That's probably your AV2 input. The yellow is composite video, and the red and white are left and right stereo sound. Whilst mainly of use for camcorders, you can get SCART adaptors (for a quid or two) that convert a scart socket into the 3 appropriate channels, and the connecting leads are pretty cheap. (Obviously you can pay a lot more for top quality kit!)
Anyway, if you have a piece of A/V kit connected up to the 3 sockets and press AV2 on your remote, you should be able to see whatever your kit is playing. If the sockets are on the front, it's going to be pretty ugly if you leave the kit permanently connected, it may be useful occasionally - such as for the aforementioned camcorders or hooking up a PC or laptop from time to time. (The latter will probably need an S-Video to 3 RCA adaptor).