RTCP or RTP control protocol is a protocol that works with RTP (Real-time transport protocol).
Whereas RTP sends media (e.g. a video stream) over an Internet connection. RTCP sends information about the quality of the stream – things like dropped packets or skipped frames – to report the status of the connection itself (and a few other things). It does not send any media whatsoever.
Informally, it's similar to the "Tools" -> "Media Information" -> "Statistics" display that the Qt interface of VLC will show about playing media.
- RFC 3550: RTP section 6: RTCP