A wireless mesh network is any wireless network where data is transmitted using mesh networking. That is, where nodes don't just send and receive data, but also serve as a relay for other nodes and each node collaborates in propagating data on the network.

A wireless mesh network can be thought of as a collection of nodes where each mesh node is also a router. Compare this to a WiFi access point where service can be provided only within reach of the signal and when it is turned off, the connection is gone. Mesh nodes work differently by rerouting data to another hop which it is connected to, bypassing the empty area where a node might be off.

Techopedia explains Wireless Mesh Network (WMN)

The concept of mesh networking can be applied to both physical and wireless networking, but it's much more commmon for wireless networks given the cabling costs that would be required to implement as a physical topology.

A key difference here is that mesh nodes work in a cooperative gain scheme where the more nodes that are active, the greater the bandwidth available Consider this analogy in traditional networking: when cars (data) coming from a wide road comes to a small bridge they all have to slow down to wait in line. To increase the number of cars going through, you need to make a bigger bridge (add bandwidth) which is then wasted in times of less traffic. In mesh networking, imagine people coming to a river, where in order to continue each person drops a rock to make a foot bridge. The more people that needs to pass, the more rocks are thrown in. But if there are fewer users, then only a few rocks are needed. In other words, the bandwidth scales automatically based on the number of users

Definition - What does Mesh Networking mean?

Mesh networking is a type of network topology in which a device (node) transmits its own data as well as serves as a relay for other nodes. Routers are used to provide the best and most efficient data path for effective communication. In the event of a hardware failure, many routes are available to continue the network communication process.
Techopedia explains Mesh Networking

There are two types of mesh networking topologies:

Total Mesh Topology: This kind of topology is in effect when every node in the network is connected to all the other nodes with direct links. This provides greater redundancy, because if any node fails, the network traffic can be directed using other nodes. Each node accesses the working nodes in close proximity and finds the best route for efficient and reliable communication. Partial Mesh Topology: This kind of topology is in effect when some nodes are connected with all the other nodes using direct links, while some are just connected to one or two nodes only. This is less expensive to implement compared to total mesh topology, but has less redundancy.

A mesh networking layout is not commonly used because of high costs related to cabling, devices and its complex infrastructure. However, wireless mesh networks are very popular among wireless networks and their users. This is because, by definition, a wireless network does not need cabling or any other physical infrastructure other than an access point.

Definition - What does Node mean?

A node is a point of intersection/connection within a network. In an environment where all devices are accessible through the network, these devices are all considered nodes. The concept of nodes works on several levels, but the big-picture view defines nodes as the major centers through which Internet traffic is typically routed. This usage is somewhat confusing, as these same Internet nodes are also referred to as Internet hubs.
Techopedia explains Node

The idea of nodes was popularized with the adoption of packet-switching theory and the concept of distributed networks. In this context, nodes were gateways that could receive, store and send information along different routes through a distributed network. Each node was given an equal standing within the network, meaning that the loss of any one node wouldn’t significantly hurt the network.

When applied to an office or personal network, however, a node is simply one of the devices that performs a particular function. As such, the loss of that node usually means the loss of function, such as the inability to use a printer.

What does Router mean?

A router is a device that analyzes the contents of data packets transmitted within a network or to another network. Routers determine whether the source and destination are on the same network or whether data must be transferred from one network type to another, which requires encapsulating the data packet with routing protocol header information for the new network type

OSI MODEL

These jobs are split into seven layers, each of which depends on the function’s “handed-off” from other layers. As a result, the OSI model also provides a guide for troubleshooting network problems by tracking them down to a specific layer. Here we’ll take a look at the layers of the OSI model and what functions they perform within a network.

1. Physical Layer
The physical layer is the actual cable, fibers, cards, switches, and other mechanical and electrical equipment that make up a network. This is the layer that transforms digital data into signals that can be sent down a wire to transmit data. These signals are often electrical but, as in the case of fiber optics, they can also be non-electrical signals such as optics or any other type of pulse that can be digitally encoded. From a networking perspective, the purpose of the physical layer is to provide the architecture for data to be sent and received. The physical layer is probably the easiest layer to troubleshoot but the most difficult to repair or construct, as this involves getting the hardware infrastructure hooked up and plugged in.
2. Data Link Layer
The data link layer is where information is converted into coherent “packets” and frames that are passed to higher layers. Essentially, the data link layer unpacks raw data coming in from the physical layer and translates information from the upper layers into raw data to be sent over the physical layer. The data link layer is also responsible for catching and compensating for any errors that occur in the physical layer.
3. Network Layer
The network layer is where the destination for incoming and outgoing data is set. If the data link layer is the highway for cars to drive on, the network layer is the GPS system telling drivers how to get there. Addressing is added to the data by tacking on information around the data packet in the form of an address header. This layer is also responsible for determining the quickest route to the destination and the handling of any problems with packet switching or network congestion. This is the layer where routers work to ensure that data is properly re-addressed before passing it on to the next leg of the packet’s journey.
4. Transport Layer
The transport layer is responsible for streaming data across the network. At this level, the data is not thought of in terms of individual packets but more in terms of a conversation. To accomplish this, protocols – which are defined as “rules of communication - are used. The protocols watch the complete transmission of many packets - checking the conversation for errors, acknowledging successful transmissions and requesting retransmission if errors are detected.

The network layer and the transport layer work together like a postal system. The network layer addresses the data, much like a person addresses an envelope. Then, the transport layer acts as the sender’s local postal branch, sorting and grouping all similarly addressed data into larger shipments bound for other local branches, where they will then be delivered.
5. Session Layer
The session layer is where connections are made, maintained and ended. This usually refers to application requests for data over the network.

Whereas the transport layer handles the actual flow of data, the session layer acts as an announcer, making sure that the programs and applications requesting and sending data know their requests are being filled. In technical terms, the session layer synchronizes data transmission.
6. Presentation Layer
The presentation layer is where received data is converted into a format that the application it is destined for can understand. The work done at this layer is best understood as a translation job. For example, data is often encrypted at the presentation layer before being passed to the other layers for sending. When data is received, it will be decrypted and passed on to the application it is intended for in the format that is expected.
7. Application Layer
The application layer coordinates network access for the software running on a particular computer or device. The protocols at the application layer handle the requests that different software applications are making to the network. If a Web browser wants to download an image, an email client wants to check the server and a file-sharing program wants to upload a movie, the protocols in the application layer will organize and execute these requests.
Putting It All Together
We’ve looked at the OSI model from the bottom layer up. A simplified summary of this process can be broken into three requirements:

The computer has to be hooked up to a network (physical layer), and must have a way to read data (data link layer). The network must also have a proper address (network layer) to know how to come and go. The network itself has to have ways of efficiently delivering data to the proper recipients (transport layer) and letting those recipients know it has been delivered (session layer). The data has to be unpacked and delivered to the application in a format it understands (presentation layer) and then must fill the requests various software applications make to the network for the user (application layer).

Sending data works in the opposite direction, starting at the top OSI layer – the application layer - and moving down through the model, finally ending when the data is received by the recipient via the physical layer.
Conclus ion: Lessons From the OSI Model

access point

In a wireless local area network (WLAN), an access point is a station that transmits and receives data (sometimes referred to as a transceiver). An access point connects users to other users within the network and also can serve as the point of interconnection between the WLAN and a fixed wire network. Each access point can serve multiple users within a defined network area; as people move beyond the range of one access point, they are automatically handed over to the next one. A small WLAN may only require a single access point; the number required increases as a function of the number of network users and the physical size of the network.

GATEWAY
A gateway is a network point that acts as an entrance to another network. On the Internet, a node or stopping point can be either a gateway node or a host (end-point) node. Both the computers of Internet users and the computers that serve pages to users are host nodes. The computers that control traffic within your company's network or at your local Internet service provider (ISP) are gateway nodes.