That's where the seatbelt would go, if your van had a rear seat. I'm not sure what is underneath the cover. Pop it off and find out. And then post a pic of what you find. (Mine has a rear seat, so I can't give a better answer than that.)
This actually houses an odd looking metal part of the van body which acts as a chamber for the sliding door rail. You can take it off but likely not without breaking the plastic cover. Attached a pic of what's underneath.