Silverskin is a membrane. It is a tough bit of connective tissue on the underside of beef and pork ribs. It can be very chewy if not removed since it doesn't get soft and melt down like fat does. Removing it also helps the seasoning to really get into the meat without having that barrier there. It can be stubborn to remove so make sure your hands and the ribs are dry or use a paper towel to get a good grip on it to pull it off.