The RPC portmapper is  a server which converts RPC program numbers in to TCP/UDP protocol port numbers. RPC must be running in order to allow RPC calls to the respective sever which could be a NFS server for example. RPC server tells the portmap on which port is it listening. and the RPC programe numbers it is prepared to serve. If any client tries make RPC call first it will contact the portmap on the particular server to know what port number to which RPC packates shold be sent.