wtf is a green glass door??

My fucking dumbass Cpl keeps telling me that shit won't go through this door. We've been in the field almost a month now and I haven't found a single thing in our bivouac that will go through this fucking door. My day pack wont go through, my peach rings won't go through, but there seems to be no problem with me carrying this .50 cal barrel back and forth. Wanted to ask here before I go to gunny