Compressor will even out the audio volume by lowering the volume when the signal is loud. This helps shape the dynamics of the sound both at the initial attack and the sustain tail:
Compressor shines on it's own, but it was primarily created to power up the different "Snapin Hosts" Kilohearts offer. These are bigger effects units that utilize all the available snapins and give you a really fun and creative workflow for combining them in any way you can think of. So have a look at Snap Heap and Multipass right now.
Compressor is also available as a Rack Extension for Reason over at the Propellerhead shop.