If you serve food au naturel, it's uncooked and plain, and if you attend a party au naturel, you don't wear any clothes. In either case, the thing that's au naturel is in its natural state.
The French phrase au naturel is used in cooking to describe something prepared very simply, and it can also describe a person with few affectations. You might, for example, admire your friend's casual, au naturel style, with no makeup or elaborate hairdo. In English, though, it usually means "naked." The literal French meaning is "in the natural state."