That is, for a connectionless protocol. The server tstSrv.tcl is not that useful for real life problems, because it blocks while servicing clients. But is fairly easy to understand. The main different between the client socket and a server socket is that the application have to enter an event loop. A server program is therefore not as straight a client program.
Every server socket has to have a command to call back when a request arrives.
At line 29 the channel is created. This option have a necessary argument, namely a callback command. This is created on line 12-25. When a client connects this client will be handed over to the callback command, in this case a procedure. The procedure is rather straightforward.