There is a shiny white, cellulose membrane on the back of ribs sometimes called silver skin. This should be removed while you are cleaning and trimming your ribs. As opposed to fat, this membrane won’t break down while cooking the ribs and leaves an undesirable chewy-ness to the rib. Once removed, this will also allow your dry rub to penetrate better since it will actually be on the meat.
Depending on where you buy your ribs sometimes they may already be "skinned" which means they're already removed the membrane for you. If the membrane is on you'll see a smooth and shiny layer on the back of the ribs.
Removing the membrane is simple and easy to get the hang of. Flip the ribs over so you are looking at the cupped-side or back-side of the bones. You’ll notice that the ribs sort of taper off to one end where. Start from the other larger end or in the middle and work the end of your handle or butter knife under the membrane. Take your time and pry it up without cutting it. Once you lift it up enough to pull it then grab it with a paper towel and pull it completely off. It should come off in one piece.
Now you're ready to apply your rub and start cooking.