port map dynamically assigns port numbers for network services such as nfs. it was invented by sun microsystems to get round having to register well know port numbers (it takes years for other vendors to catch up even with it's official).
you don't need to use it as such. it's automatic.
Portmap is a server that converts RPC (Remote Procedure Call) program numbers into DARPA protocol port numbers. It must be running in order to make RPC calls.
Services that use RPC include NFS and NIS.
google > portmap