A mop is a tool for cleaning a floor. Most mops have a long handle and a sponge or bundle of absorbent strings on one end. If someone says that your hair looks like a mop, that's not a compliment.
When you use a mop, you mop the floor. Mops can be used dry, but more often they're dunked in a soapy bucket of water before they mop the floor. You can also "mop up" a mess or a spill, even if you're using a cloth, a sponge, or a paper towel instead of a mop. The earliest mops were used to clean ships' decks, and the word comes from the Latin mappa, "napkin."