The “skin” is a papery membrane (technically known as the pleura) found on the inside or concave side of a rack of ribs. The easiest way to remove it is to loosen it from a couple of the ribs by inserting a slender metal object, like the tip of an instant-read meat thermometer or a clean Phillips head screwdriver, under the membrane, then prying it away from the bones. Once you have the membrane loosened, grab it with a dry paper towel or dishrag (the membrane is slippery) and pull it away from the rack. The skin comes off of baby back ribs very easily; you need to work a little harder to remove it from spareribs and beef ribs.