Planes are the layered worlds existing beyond our material realm, each governed by its own physical and moral laws — elemental planes of fire, water, and air, outer planes of the heavens and hells — across which characters travel by magic.
The notion of worlds with wholly different laws lying adjacent to our reality runs straight into cosmic horror’s vision of indifferent outer gods seeping through the seams of space and time. Elements like the planes and the mind flayer are the passage joining dungeon fantasy to the Lovecraftian tradition.