The Upper and Lower Planes

Their are multiple realities or universes which overlap with each other, but do not usually directly interact. To explain this relationship to new students, scholars often start out with an over-simplified example, describing each reality as a separate sheet of paper. Those sheets are then combined to form a book, in this case the "multiverse" which encompasses all realities.
This simplified mental model has led to each reality/universe being described as a "plane", though most (including the universe which contains Norath) are fully three-dimensional. Although the universes overlap throughout their full set of dimensions, mages often refer to certain sets of these planes as "upper" and "lower".
This categorization by mages is due to the fact that, if a portal linking the planes is opened, magical energy naturally flows one way across the portal,. This is likened to water always flowing from higher elevations to lower ones, unless something expends energy to force it back up.
The planes from which magical energy naturally flows "down" to Norath are described as the "Upper Planes". The Upper Planes are also the realities where the gods reside. Planes to which magical energy naturally drains away from Norath are described as the "Lower Planes", and are home to powerful beings known variously as devils and demons.

RPG Campaign Note

If an RPG source book states that a "god" resides in a Lower Plane, he/she will be relocated to an Upper Plane. This includes the "underworld" described in many mythologies. Although the path taken by mortal visitors to reach this realm may involve going underground, and the terrain may resemble caverns, in this universe the home of a "god" always in one of the upper planes.