SAP stands for Session Announcement Protocol. It's defined in RFC2974. It's a distributed directory of announcements of streams and uses multicast to efficiently distribute these announces on the Local Area Network or on the MBONE. It uses SDP's to describe streams so that you can open the stream via RTSP.
This technique will allow you to have a lot of server-generated streams (often multicasted) that announce themselves on the network. The clients on the network can then listen for these announcements. A client can receive a listing of all these streams and can simply 'tune' into an individual stream.
It can also work for Internet telephony (SIP) for instance.
Example of what is announced via SAP on the MBONE: http://www.uninett.no/multimedia/streamingguide/alle.html