RPC works like this:

A machine has a bunch of services that need to run, only they don't have a port assignment. Hmmm, tough luck? Not really. Your system assigns these services a random port number that is not in use and then registers it with the RPC service. This allows the remote machine to connect without really having to know many network details.

Along comes your machine on the network, and it would like to access one of these services (or programs). Your machine asks the RPC service where it can find the service it is looking for. RPC responds to your machine, which then connects up to the service based on the info that it received from the RPC service. When I learned how RPC works, my mentor told me to think of RPC as the information booth at Grand Central Station. As you can see, I still explain it the same way.

This is a very simplified explanation. Many technical things happen during the above process but I'm not exactly in the mood to write a technical tutorial on RPC.

Hope this helps you out.