That's the in bounds trigger. In Marble Blast you are not meant to leave the level (since otherwise you'll fall off indefinitely) so it surrounds the level. In three levels the trigger wasn't scaled properly so you can fall off indefinitely, but also make a funny glitch where you are within the trigger, restart and since you aren't in the trigger upon restart, it calls you out of bounds and then it restarts the level normally.
Matan