a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level

RFC3550 defines the capability to extend the RTP header. This line defines extensions which will be used in RTP headers so that the receiver can decode it correctly and extract the metadata. In this case the browser is indicating that we are going to include information on the audio level in the RTP header as defined in RFC6464.