The Astral Sea is a Fundamental Plane that connects all other planes together.
The plane is an ocean of silvery liquid with the stars visible in distance. Some of these stars are just physical locations in the sea itself. Others are planes unto themselves, islands floating in the Astral Sea, stepping onto the shore of one will transport one into that realm.
The Astral Sea touches the mortal world in one place, atop a mighty mountain. This weak spot between the worlds is guarded by the Marut stationed at the Astral Fortress.
The Astral Sea was created by Ra'Ver-Alec and Ator in ages past. It is the home plane to the Shadar-Kai, for Ra'Ver Alec created and nourished them here. It is also from this plane that the god transported an entire continent to Galara.