RPC stands for Remote Procedure Call. It refers to the use of a programming language procedure (aka function, method, etc.) as an abstraction for request/response style network messaging. Since most programmers are familiar with procedures, this makes network programming very easy. Take the following procedure call:

t = getCurrentTemperature();

This may look like a normal procedure call, but under the covers, it's really connecting to a remote machine across the internet to get the weather. All the complexity of TCP/IP is hidden behind a simple procedure call.

There are many different version RPC on the market: RMI, CORBA, DCOM, SOAP, XML-RPC, to name a few. The behind-the-scenes details may differ but the concept is the same: hide the details of network programming behind a simple a function call.