Forward error correction adds redundant information to the stream so the receiver can rebuild lost packets without asking for a retransmission — which real-time audio has no time to wait for. Opus's in-band FEC carries a low-bitrate copy of the previous frame inside the current packet, so a single lost frame can be recovered from the next one to arrive; the RED scheme (RFC 2198) instead bundles whole previous payloads alongside the new one. The cost is bandwidth: redundancy only helps if you spend extra bits, and it only protects against the loss pattern it was sized for. FEC and packet loss concealment are complementary — FEC recovers data, PLC hides what FEC couldn't.
