FID denotes the Flow Identification which is used in the SDP for describing the relationship between the primary media SSRC and the retransmissions SSRC.
You can find the definition and example for it in RFC 4888 section 8.7.
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The WebRTC Glossary is an ongoing project where users can learn more about WebRTC related terms. It is maintained by Tsahi Levent-Levi of BlogGeek.me.