Rilutaan: The Western Marches
The Astral Plane is one of the Transitive Planes. It, along with the Ethereal Plane, allows access to other planes of existence. The Astral Plane in particular has access points to all other planes, and can be used as a go-between for the world and the Upper and Lower planes, as well as other, more obscure places.
Travel within the Astral Plane can be disorienting, as visitors are able to move freely in all directions. The astral projection spell is a common means of visiting the plane, and one that can be safer as it projects the soul into the plane instead, though that does carry its own risks.
Much of the Astral Plane is empty, though occasionally there are chunks of floating, solid matter that can be encountered here, of varying shapes and sizes. Some of these floating islands are large enough to support structures and life, up to and including immense cities and citadels of all manner of beings.