Pork ribs have a thin membrane that runs along the back of them. Sometimes you can buy ribs with it removed, but often they come with the membrane still attached.
Removing the membrane will make your ribs more tender and allow your marinade to penetrate the meat more effectively. It's a simple step that will improve your dish.
A run of pork ribs (in this case back ribs). You can tell the membrane is still attached as the underside is a little shiny and none of the meat underneath the membrane is visible.
Insert a pointy object (I am using a chopstick) just under the skin of the pork and pull up. The membrane will lift up enough for you to grab hold of it.
Grab the membrane and pull. If it's a little slippery, hold it with a paper towel. This will give you a tighter grip.
The pork ribs are now sans membrane. The difference is subtle, but once removed, the surface of the back side will be slightly rough and you may see a little bit of the flesh underneath.