Astral Plane
The Astral Plane is one of the Inner Planes of the Great Wheel cosmology - in fact, it stands opposed to Prime Material and/or Ethereal Plane in that it is the outermost of the Inner Planes, standing as the final buffer between the Elemental Planes and the Outer Planes beyond.
The Astral Plane is, for the most part, a dreary and empty expanse of nothingness, broken up upon by sporadic landmasses, the petrified corpses of dead gods, the gargantuan worm-like "astral vortexes", and color-based portals to various planes. Gravity works weirdly, as you can essentially "pick" which way is down at any given moment.
There are a handful of sapient races indigenous to the Astral Plane, but the most well-known are a bunch of immigrants, the Githyanki. Living creatures don't age here, so the githyanki need colonies in the prime material plane for their eggs to hatch and their children to grow up. They also build ships and cities out here, the biggest on the corpse of a dead god that their leader is trying to eat.
Lots of people who go here prefer to travel via "astral projection," leaving their physical body behind and letting an astral body glide through the plane. This is generally fairly safe, but they need to keep track of their "astral cord," connecting their astral and physical bodies, which manifests as a very tiny silvery string. If this gets severed (which not many things can, but it is the intended purpose of those silver githyanki swords), you're toast. If it's not, you can tug on it to get back fairly easily.