This question might get the most response in the Newbies section. In short compression is adjusting the dynamics so there is more evenness between the soft and loud parts. It does get MUCH more complicated than that unfortunantly and compression goes way beyond loud and soft and can drastically change the way something sounds. It was one of the hardest things for me to grasp about audio for a while. It's not like slapping reverb or delay on a track. It's a very subtle balance between science and art. Hope this helps.
