It could be two things: the drain pan or the defrost drain. First off, check at the lower back side of the ref. On top of the compressor lies the drain pan. If you see that it is overflowing, just take it out (carefully) and throw the excess water away; replace it if it's damaged in any way. Second, check the defrost drain. This is the hole at the back of the freezer. Remove any blockages in the drain hole; if it is iced over, defrost it.