DIAGNOSIS This is usually defrost water that isn't draining properly. During normal cycles, frost melts off the cooling element and the water is supposed to run down a channel to a evaporation pan near the compressor. That channel is likely clogged.
FIX You need to clear the drain path. 1) Unplug the unit and empty it. 2) Remove the shelves and the plastic drip tray. 3) Look at the back interior wall, near the bottom. You should see a small hole or channel. That's the drain. 4) Using a turkey baster or a long, flexible pipe cleaner, gently flush warm water into the hole and try to clear any debris. You can also use a small amount of diluted vinegar to break up mold or sludge. 5. Dry everything thoroughly before reassembling.
IF THAT DIDN'T WORK If the water is still appearing, the drain tube (inside the cabinet) might be disconnected or kinked. This requires removing the rear service panel to inspect—a more advanced step.
HEADS UP Don't ignore this. Standing water inside will lead to mold growth and a bad smell. It also makes the compressor work harder to cool a humid environment. Make clearing the drain part of your yearly maintenance.