Say youve got a bass guitarist that plays really hard, and then soft all in the same passage or bassline.
Youve decided that the only way this bass is gona sound good is if it sounds nice and even.
So you put a compresser on it. (either a plug in or a hardware unit)
When set correctly, the compresser keeps the hard parts down in volume (by "compressing" them) so now the hard parts are the same volume as the soft parts.
Does that make sense?