I believe they are flaps that cover some of the bolts for the OEM roof rack. They're just there to look nice and cover the bolts. It goes w/o saying there should be 4 of them. The rack should work just fine w/o the flaps but it would be nice to have all 4. Firstly, it gives it more of a completed look but also with them hiding some of the parts it helps just a little to keep the rack yours and not someone else's.