RPC stands for remote procedure call. The basic idea is that programs call functions on remote programs (identified by program numbers), and the remote programs return a result code or message.
RPC implementations use transport protocols such as TCP and UDP, and they require a special intermediary service to map program numbers to TCP and UDP ports. The server is called portmap, and it must be running on any machine that wants to use RPC services.
If you want to know what RPC services your computer has, run this command:
rpcinfo -p localhost
RPC is one of those protocols that just doesn't seem to want to die. The unpleasant Network File System (NFS) and Network Information Service (NIS) systems use RPC, but they are completely unnecessary on stand-alone machines. But whenever you think that you've eliminated all need for portmap, something else comes up, such as File Access Monitor (FAM) support in GNOME.