The rib membrane is a water proof film that separated the chest cavity of the hog from the muscle/rib area. It keeps virtually everything from passing in or out of the chest area. When cooking ribs this means that it keeps out smoke and seasoning flavor and holds in the fat that needs to be rendered. This is why I remove the membrane. Plus it is really easy to do and only takes a few seconds once you can get a good grip on it.
You can see the membrane as a dull, whitish film over the rib section of the rack.