At the risk of sounding like a pompous ass, I'm going to suggest that this should have been posted in the newbie forum. That being said, you may want to do a search for 'compression', maybe in the title only. There is a ton of info for you.
do the search thing for details but hears a quick idea. compression is basically automatic volume control. it effectivly reduces the volume of the loudest parts which in turn cuases the quieter bits to appear louder. for instance on a tom compression would reduce the volume of the initial hit and "increase" the volume of the toms tone, "fattening" the sound. this is very simplified!!!!