Martinman,

Compression is a tricky thing to wrap your brain around and even trickier to master the use of. The RNC is a wonderful unit and can be used with great results on vocals.

Let's see if I can boil it down: the basic idea is to "smooth out" level fluctuations so that there is less of a difference between the loudest and quietest moments. This will allow you to bring the overall level up. In essence the loud moments get quieter, the quiet moments, louder. Tracks can be treated this way to sound more "up front".
Because the ear naturally compresses loud sounds (ever put your head near a snare drum?) compression can give the illusion of power.
If used properly (or traditionaly) you really shouldn't hear to much of an overt effect. When overused compression will create an unsettling "pumping" or "breathing" type of effect in a signal.

The RNC shows you how much of an effect it is having with the row of red lights that read "gain reduction". These tell you how much the level of the signal is being reduced. The higher the threshold is set, the louder a signal has to be before being "squashed". The higher the ratio is set, the more it is squashed....
Play around, it can be alot of fun. The best way to learn is to push play and start tweakin.

Well, I tried. Sorry this is not as eloquent or susinct an explination as I would have liked. Good luck.