Yeah, there's cooling coils down there, but there should also be a very shallow plastic or something pan that is under the coils and may run most of the depth of the refrigerator. It collects water that drips off the coils, with the intent that the water will evaporate off the pan.
Sometimes it collects faster than it can evaporate (like in the summer humidity) and there's standing water in the pan that starts growing mold in it.
Somebody posted a picture of one here
http://thefullerbrushgirl.com/blog/w...7/drip-pan.jpg