Run water through the copper coils and submerge into your boiling brewpot. Chills your hot wort quickly for a better "cold break." The key is to trickle the water through slowly so that the cold tap water comes out blazing hot - that is your "heat exchange." If you run the water through too quickly, the water goes in cold, comes out cold and you have no energy removal. And you can waste tons of water.
25 foot is good for 3-5 gallon boils.
50 foot is good for 5-6 gallon boils.
Larger volumes than that are best chilled with counterflow chillers.