A mooring is kind of like a parking lot for boats — it's a place people can keep their dinghies, sailboats, or ships tied up.
One complication of owning a boat is that you need to rent a mooring for it, a place to keep it while you're not out on the water. Piers, docks, and wharfs all typically provide moorings for boats and ships, where they can be attached securely with ropes. Mooring comes from the verb moor, "to fasten by cable," from a Germanic root.