This is the little flat black plastic thing you see sticking through the trim on the front door and helping to hold the plexi glass in place. These are the same type Retainer clip parts that are used in many cars and vans to hold carpeting in place, just probably a different size.
In order to remove the existing ones when you are replacing the broken plexi glass simply drill through the center of them with a drill bit almost as fat as the tree part that sticks through to the other side. Once the head has come off like a plastic rivit, then grab the back side with wire cuttes, dikes, or pliers and pull it on through. New ones are inserted by pushing through the trim, and are used to hold the plexi glass in place so it does not flex out during a storm. It is also an easier way to hold the trim in place and aligned as you put the screws back in some types.
These come in sets of 3 so if you order a qty of 2 you will get two sets or a total of 6. Some People also call these Spruce Tree or Pine Tree clips.