The Prime Material Plane is the plane of existence that contains the Natural World, the Feywild, the Shadowfell, aswell as the moon, and the sun. It's location in in the universal cosmology is generally considered to be "below" the Astral Sea, and "above" the Elemental Chaos. Such directions are of course meaningless in the grand scale of the cosmology, but some find them to be a helpful indication of the Prime Material Plane's relative place in the universe.