A maw is a hole in the background of a world through which various hostile mobs, or even poison gas (in deep biomes), may emerge.
Placing a plug over a maw will close it off so mobs can't escape. Plugging a maw earns 5 XP.
If you are unable to craft plugs to seal off a maw, you can place blocks around it to trap hostile mobs.
In the Deep Biome, culverts explode, emitting a poisonous gas that is often lethal. The explosions also deal high damage. They cannot be plugged until after the world has been purified.