The Moonpool was a big pool of water that the moon fell in. The Clan cats drank out of it because it was laden with alcohol, and they LOVED IT.
When the Moonpool was first found by the treacherous drug addict, Leafpool, her system was full of meth as she stumbled to the Moonpool, and she saw the moon crash into the alcohol infested waters. She screamed to all the cats she knew, "THE MOON CRASHED INTO OUR ALCOHOL!" All the cats screamed in terror as a kit drank out of the water. "Mmmm. Tastes good!" And that is the story of the Moonpool.