I know I won't have a good way of explaining this, so here's my shot at it...
"Gating" generally means to cut a portion of the signal out of the sound. Think about hitting a snare drum in a concrete basement--long after the initial "thwack", you're going to hear a massive amount of reverberation. If you "gate" that drum signal when you're recording or mixing, you're looking to chop off some off the nasty ding noise that makes the drum sound like...well, like you recorded it in a concrete basement. You might have also heard the term "gated reverb", in which you would add reverb to the signal in order to prolong the "thwack", and then gate off the end of it in order to get a very thick drum sound.
Gates are also used to get rid of some of the drum "bleed" that occurs when your using several mics to record a kit. You might only want the snare mic to pick up just the snare, but chances are good that you're going to hear some of the toms or other drums in that mic as well. By setting the gate to letting only the sound above a certain threshhold (i.e.--"I just wanna hear the loud one"), it will help to get rid of some of the other sounds that you didn't want creeping in there.
Hope this helps. I know other people can explain that better than I just did, especially since it's likely that they will actually know what they're talking about.