There's nothing more annoying in a game than cruising across a vast open world, enjoying the sights, and completing quests, only to then crash into an invisible wall. Developers use invisible walls to contain the player in the world that they have created, ensuring that gamers don't go out of bounds and enter areas they're not supposed to. The name comes from the fact that, traditionally, these sorts of blockades couldn't be seen with the naked eye, meaning that players would literally be boxed in by something that didn't seem to exist physically.