The panel enclosure, the metal box itself, needs to be bonded to ground so that it cannot become energized by a loose wire, for instance, and become a shock hazard for the next person who touches it. You are looking for a green screw in the grounding bar that screws into the back of the enclosure, or a copper connector from the ground bar that screws into the enclosure, and I'm probably forgetting some other things. I'll look for photos.