It is a small amplifier, generally run off a 9v battery, that boosts the signal from the mic to line level. thus enabling the mic to be used on a "line in" jack. I check and reactivesounds doesn't actually preamps. Only battery boxes. Battery boxes provide extra power to the microphone. This ensures that the mic is working at its full ability and dynamic range, it does NOT however boost the signal coming from the mic.
Check out this FAQ:
http://www.soundprofessionals.com/micacces...batteryfaq.html
Hope this helps,
bilbo