When you compress an audio track, it brings up the volume of the quietest moments while lowering the loudest moments. It essentially squeezes the signal so that an instrument can be perceived in a more pleasing way to the ear. When applied properly, compression can bring something forward in the mix while smoothing out the sound.
A general tip about compressors: think of it like adding spice to your cooking. No need to dump all the salt you have on your chicken! Instead, sprinkle a touch of salt and then continue cooking. Perhaps you'll add a little more when you feel it needs it. In more direct terms, don't overdo it! If you do want a lot of compression, try using several compressors in a row so that one isn't doing all the heavy lifting. :)
