RTP is the Real-Time Transport Protocol, used to transmit voice and video. RTCP is the RTP Control Protocol, used "to monitor the quality of service and to convey information about the participants in an on-going session." Here's a lot of documentation. RTP and RTCP are Internet Standards and are defined in RFC 3550 and RFC 3551.
RTP is usually run over UDP, although it can be used over other protocols. The standard specifies that RTP data should be carried on an even port number and the corresponding RTCP packets should be carried on the next higher (odd) port number.
l7-filter uses the rtp pattern. It has been tested and is confirmed to work for at least some programs. Since this pattern is by necessity rather permissive, it is recommended that it only be used after packets have been checked to have even source and destination ports.