The Astral Sea, known also as Astral Plane, is the plane of thought, memory, and psychic energy as well as one of the two fundamental planes. Held tightly within it are the dominions of the gods, as well as the Nine Hells, while the open spaces in between it is where gods go when they die or are forgotten and what was once mistaken for the Astral Plane in its entirety, a barren place with only rare bits of solid matter. The Astral Sea is unique in that it is infinitesimal instead of infinite; there is no space or time here, though both catch up with you when you leave. The souls of the newly dead from the Prime Material Plane pass through here on their way to the afterlife or Outer Planes. 1
1 Taken from http://forgottenrealms.wikia.com/wiki/Astral_Sea.