It's something that goes between the axle and (typically) the frame. Usually it has a little bit of squish to it as well.. harder than plain rubber, but softer than steel.
The idea is if you fully compress the springs, the bump stops will prevent things like:
The tires from hitting the body
hard parts on the axle from hitting the frame
Stuff like that.. to "cushion" the impact...
Look under yours, I'm assuming you have a Jeel YJ from your name.. look on the bottom of the frame, above where the axle tube is.. you should see some rubber things sticking down.. They come in all shapes & sizes...