Ooh! I just had this problem. It turns out that your freezer has a little hole in it that leads to a tube that drains into a pan underneath your refrigerator. This is so that condensation can drain out, rather than freezing inside (it is how the refrigerator defrosts itself).
If you have water collecting in the bottom of your refrigerator, that can mean that your little defrost hole in the freezer is clogged (with ice or whatever). If you can take the bottom of the freezer out (it's like a big piece of plastic that is screwed on) with a screwdriver, you should be able to find the little hole. Use a turkey baster and dribble some warm water and baking soda in there. It should melt the ice and clear out the pipe.
Then, the water will drain where it's supposed to rather than backing up inside your refrigerator! Good luck!