The most common cause of water pooling in the bottom of a refrigerator is a clogged defrost drain tube. In a side-by-side refrigerator, the clogged tube often also results in ice forming at the bottom of the freezer.
Refrigerators with top-mounted and bottom-mounted freezers may need some disassembly to clear the clog, and this procedure should be performed by a service professional. In the case of side-by-side units, the drain hole is often more visible and easily accessed. If the drain is accessible, it can be flushed by forcing a solution of 2 cups of hot water and 1 teaspoon of baking soda into the tube.