There is a rock in one of the wheels. It might be because a rover doesn't have pockets.
Someone has picked up a great rock, carried it around, saved it, or placed it somewhere special. The Perseverance rover has been carrying a random Mars rock in it's wheel for a long time.
There was a rock in the left front wheel of the rover. The rock has been with the rover for a long time. The rock isn't doing any damage, and the team thinks it has traveled more than five miles from its original location.
There is a potential down-side to the rock in the wheel.