Navmesh

A Navmesh is a mesh used by AI, like enemies in an FPS level to know where they can go, where they can't go, and can be used to help them navigate through places in any way you would like. It exists as a very useful tool when designing levels in Unity and Unreal-Engine 4 because it allows you to design the level without having to think about exactly where the enemies can and can't move, and instead think about the parameters the navmesh can cover. Once you have it configured, you can have it scan the space of your level and bake the navmesh into your level. It will create it so that the NPC's can move freely in the spaces where you want them to, all without spending hours implementing your own custom navmesh. You can still create or edit the algorithm-generated navmesh, but for any large levels and games, doing it by hand is unrealistic