PLI stands for Picture Loss Indication. It is one of the error resiliency mechanisms in WebRTC.
A PLI can be sent when the receiver of the media lost a full frame or more. It is sent over RTCP.
A receiver of this message can decide to resent a full frame if he has one available in cache and finds it usable.
This message is similar to a FIR message but a bit more lenient in its heuristic.