A leak is where the "inside" lighted area where players reside has an opening (it can be large or 1 unit^2) to the black expanse outside the map. You will know there is a leak if it says ****LEAKED**** in the black box.
After you fix the leaks, the lighting of your map will change a bit. If you want to have the bottom of the caverns still dark (like a black expanse), you will have to use negative lights, which I believe ArghRad supports. Instead of emitting light, they take it away. Like anti-light.