Large cuts of meats such as butts, tenderloins, ribs etc have a thin membrane surrounding the muscle called silver skin. Unlike collagen, silver skin does not dissolve during the cooking process and should be removed. If left intact, it will toughen during the cook.
The removal process can be somewhat daunting but with practice it will become “second nature”. Simply slide the blade of a very sharp knife under the membrane working along under the silver skin trying not to remove too much meat.
When it comes to ribs, the silver skin is opposite the meat side. Using a knife will not effectively remove the membrane. On the wide side of the rib, carefully slide a knife under the silver skin and try to pry it up so you can grab it with your fingers. You will need a paper towel to help grab the membrane so it won’t slip out of your fingers. Pull the skin back off the rib.