SDP stands for Session Description Protocol. It is defined in IETF RFC 4566.
SDP is used by WebRTC to negotiate the session’s parameters. Since there is no signaling in WebRTC, the SDP created and used by WebRTC is assumed to be communicated by the application and not by WebRTC itself.
There is a lot of criticism related to SDP, which is one of the reasons that the ORTC initiative was created.