Wireless links can be very unpredictable and subject to fading, especially in situations with a lot of objects like buildings, vegetation, or vehicles that can attenuate, reflect, scatter or diffract signals. So, in average, a receiver gets several versions of the signal, all having possibly traveled different paths and therefore having different phase offsets.
When these signals combine, they could either add up (if they are in phase) or cancel each other out.
Thus, the signal quality at the receiver heavily depends on the geometrical properties of the environment. When considering moving objects, this situation is also changing, typically in the order of some 100 milliseconds.
So, in one moment, there could be a good connection from A to B, while for the next transmission this connection is bad. Cooperative relaying is a method, where other nodes in the network are used to assist the transmission.
This scheme is supported by the facts that (i) more and more devices are equipped with wireless transmitters, and, (ii) a wireless transmission is basically a broadcast communication that reaches several nodes at once.
For example assume, A wants to send a message to B, but the channel A->B is bad. Within the sending rang of A there are also three more nodes, C, D, and E. Let’s assume, C and E could receive A’s message to B well (the channel A->D was bad at this moment). So, C and E have now a copy of this message. B can now choose between two nodes to get the message forwarded, a much higher chance than asking A to resend over the probably still bad A->B channel.
Coding theory allows for even more sophisticated schemes where the message is transmitted in part over several diverse channels. However, the basic principle stays the same: utilzing diverse transmission channels can bypass the fading problems in wireless communications.
See this overview paper for more information on cooperative relaying:
W. Elmenreich, N. Marchenko, H. Adam, C. Hofbauer, G. Brandner, C. Bettstetter, and M. Huemer Building blocks of cooperative relaying in wireless systems. e & i, Springer, 125(10):353–359, 2008