The port mapper (rpc.portmap or just portmap, or rpcbind) is an Open Network Computing Remote Procedure Call (ONC RPC) service that runs on network nodes that provide other ONC RPC services.
Version 2 of the port mapper protocol maps ONC RPC program number/version number pairs to the network port number for that version of that program.
When an ONC RPC server is started, it will tell the port mapper, for each particular program number/version number pair it implements for a particular transport protocol (TCP or UDP), what port number it is using for that particular program number/version number pair on that transport protocol.
Clients wishing to make an ONC RPC call to a particular version of a particular ONC RPC service must first contact the port mapper on the server machine to determine the actual TCP or UDP port to use.
By using a spoofed port mapper request, an attacker can amplify the effects on a target because a portmap query will return many times more data than in the original request.