Locations
Imagine that the world as you know it and all the stars and planets in the sky are but a single infinitesimal point in the grand scope of all known realities, a place called the Material Plane. Stretching out in two directions are two cones, spreading out wider and wider as they get farther from the Material Plane. In one direction, what we consider to be 'outward', there is the Astral Sea and all the Outer Planes, those planes of existence which are the manifestations of the mental and astral selves of those inhabiting the Material Plane. In the opposite direction, the inward, we find the Ethereal Plane and the Inner Planes, the planes which are manifestations of the physical world. Beyond those planes, at the innermost point of the Cosmology, is the Plane of Elemental Chaos, just as infinite and inimical to life as the Realms Beyond The reason that this Cosmology is described as 'like an hourglass' is because these two directions ('outward' and 'inward') are most easily illustrated as two directions (either left and right or up and down) on the page and the number and size of planes increases as you get farther from the Material Plane.