A sound module is a synthesizer which doesn't have a keyboard. So you connect a midi controller (ie. a midi keyboard) to the sound module and it plays the sound. Sound modules typically feature a lot of samples of real instruments as their source (ie. samples of brass, piano, etc...), so you get realistic sounds right out of the box. Sound modules also tend to have a lot of 'preset' sounds in them to get you going quickly, which is nice to use as a starting place for creating your own synth sounds.
I know this is a very simplified explanation, but since you're just starting out, I didn't want to overwhelm you with the details.
Some of the popular sound modules that beat makers use are:
Roland jv2080 or jv1080
Korg Triton rack or Trinity rack
Emu Proteus 2000
Emu Planet Phatt or Mo Phatt
You'll find at least one of these in nearly every studio.
-=bumpus=-
