What is a audio compressor?

A compressor allows you to compress your audio and bring out things that aren't as loud without making the things that are loud any louder. Essentially you are taking the dynamics away from the music. Depending on the genre this can be good or bad. In classical music there are a lot of dynamics. When the entire orchestra is playing the music is loud, but when it breaks down and its just the flutes playing it gets very quiet. Using a compressor you can make that louder. In this scenerio you wouldn't want to do it because that's a part of Classical Music having that dynamic range. When you come to Pop Music everything is compressed to hell to make the track as loud as possible. On an individual track such as a vocal track you can use a compressor to even everything out. So that one part of the recording that the artist doesn't sing as loud can be compensated for without having to re-record the entire track.