The bib screw has backed out and the faucet washer is loose. The screw can get caught between the washer and the seat and ruin the faucet. Or sometimes it sticks in there and the water won't turn off. Usually when you are not at home and the person there doesn't know hot to turn the water supply valve off.
PS.
Don't have too much fun with it, the resulting water hammer can break pipes, hangers and cause other things in the house to leak.