The membrane of pork ribs lies on the interior side of the rack. It is edible, and some barbecue cooks do leave it on when they prepare the ribs. Most do not because seasoning will not penetrate the tough membrane. The best way to remove the membrane is to cut a small piece of it with a knife, then pull it out with your fingers.
Slide the tip of a paring knife underneath the membrane along the widest end of the ribs, right next to the first bone. If you are working with baby back ribs, start underneath the second bone of the rack.
Carefully rock the paring knife back and forth until there is a space big enough to insert a thumb. Gently pull up the membrane until you can insert two fingers.
Pull the membrane off of each bone until you reach the end. The membrane should come off in one piece. If the membrane breaks, slide the knife tip underneath the next bone, and start again until all the membrane is removed.