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.
Honestly, I has not been able to totally understand the mslabel attribute. Looking at this mail thread I understand that is a deprecated attribute which a function similar to msid.