Behind your refrigerator near the condenser fan there should be a pan that collects water from your refrigerator. The water is supposed to evaporate but sometimes it doesn't.
The waste heat from the condenser is supposed to evaporate this water. First make sure you condenser fan is working. If it's not then the water will overflow and leak out. If the condenser fan is burnt out you'll need to replace it or get it replaced.
If that's still not it check for a clogged drain line. This could be causing it to overflow. Or even a cracked drain pain could be causing the water to leak behind your fridge.
If you have an ice maker make sure the water line isn't leaking.