If your pork tenderloin has not been trimmed, you should remove the silver skin. It is a silvery sheath of connective tissue that does not break down or soften during cooking. It's easy to trim silver skin with a sharp knife. Just cut into it and lift it up, carefully slicing down along the tenderloin to remove it. If you don't see silver skin, it just means that it was trimmed by the seller - lucky you! Many times I find the pork tenderloin in the 2-packs come beautifully trimmed and ready to cook.