Next we have the ssrc related lines. SSRC stands for Synchronization Source and it is an unique identifier of the RTP media stream source. In this we have many sources. Roughly speaking, these lines establish a relationship between the SSRC flow identifier - an older RTP concept – to a WebRTC Media Stream – a newer WebRTC concept.
This line is used to signal the association between the RTP concept of SSRC and the WebRTC concept of “media stream” / “media stream track” using SDP signaling [draft-ietf-mmusic-msid]. As you can see, the first number is the SSRC identifier that will be included in the SSRC field of the RTP packets. Following msid: we have the unique identifier included in the semantic.