compresses your sound so that the highs arent too high, and the lows arent too low. It levels out the volume contour.
It reduces the volume of high sounds and/or increases the volume of low sounds, to make everything sound more even. A byproduct is that sustain is also often increased.
go to like guitar center.com .. check out a compressor and listen to a audio file.

its hard to explain
a compresser makes it so that every note you play is exactly the same volume, e.g if you play a note softly it'll come out the same volume as if you played a note hardly, if that helps