Jounce bumpers and bump stops are hard foam and rubber parts that absorb sudden blows as a vehicle travels over potholes, curbs, and bumps in the road. They fit over strut inserts, and shock absorbs. They're fastened to control arms and leaf springs to prevent them from fully compressing metal to metal when the vehicle bottoms.
Replace jounce bumpers and bump stops whenever they're worn or missing. They dry out in time requiring replacement, but unfortunately do not always come with replacement parts like strut inserts. Symptoms of missing and worn bump stops include noise and vibration caused by the metal to metal contact.