The Astral Plane is one of the Transitive Planes of the Great Wheel cosmology... Wait, that's not entirely correct. In the most technical sense, it's not even actually a "plane" in the sense of a bound dimensional space like the others.
Light up a bowl, kids, being intoxicated might actually help with understanding this one.
The Place That Isn't
Per the original 2nd edition supplement A Guide To The Astral Plane, the whole place is essentially the empty place between everything. It's not a place the way you can actually think of it; it's actually the absence of normal space and time, being a realm of pure concept. It's a realm between concepts. If you think of all the Outer Planes representing certain cosmological ideals and metaphors and such, the Astral Plane is simply an area where concepts haven't taken a form yet. It is the infinitely small space between everything, which makes it infinitely big as well once you get there.
Confused? Good. Now you get the point. 3rd edition did well by refusing to categorize it as anything but a transitive plane, in other words a place between places. These attributes give the Astral some pretty unique and fun traits.
- No space means there is no gravity, and no real sense of direction relative to anything, fundamentally making the Astral a magical variety of hyperspace or related concepts. There are things that drift there in the vast emptiness, but due to the very subjective nature of distance and direction, finding such things on repeat visits can either be simple or hard as fuck. So how do you move? Pure thought, berk. Anything with an Intelligence score can fly as fast as their thoughts can take them (basically perfect fly speed of Int score x 10). Objects and brain-dead creatures can't move, but can be pushed.
- No time, so nothing ages due to metabolism not occurring. For some things, this is great: you can't suffer continual damage from things like poison or disease, you can't go hungry or thirsty, and you don't age. The problem is, you also can't naturally heal; only magic will do the trick. (In 3rd edition, the timeless trait also makes it easier to cast spells as though affected by Quicken Spell metamagic.) Now, here's the catch: all of your missed time catches up instantly when you leave the Astral to a normal time area. So, if you had a disease while in the Astral for 20 years, you just suffered 20 years of its progress at once. Same with aging. The trick is, you want to cure stuff with magic before leaving. There's a line that you'll be hungry and thirsty, too, but won't die from it as long as you start gorging yourself on meatbread or something.
These things basically make the Astral Plane the biggest wizard playground in the multiverse. Some of them just retire there, and 4th edition made that a legitimate legacy thing to do at max level, retire to the Astral and use magic to pop in from time to time on other planes.
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. 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 Githyanki silver swords), you're toast. If it's not, you can tug on it to get back fairly easily.
Things To Do
As noted, the Githyanki live in this strange place, doing raids and other mean shit. But they do have a need for trade and commerce, specifically in armaments and other military goods, and they're too stuck up to dirty their hands with digging up iron or manufacturing anything if they can help it.
One of the most prolific reasons to be here, though, is creating a demiplane through various magic spells or psionics powers. Even as the most frugal interpretation of the rules, having your own private demiplane is simply great. Aside from creating truly anonymous planar prisons for hated enemies, you can naturally build your own stronghold in such a place, or use it as an agricultural business or factory site to assemble items, or whatever.
Another great thing, because the Astral touches everything, you can spend time searching for links to other planes, anywhere.
So what's the problem?
The problem is that the Astral is, as stated before, a non-bounded infinity, and thus you need to both want to leave and have a means of doing so in order to do so. Various editions of D&D have had various means of bringing this about, and the most common (and cheap) of them involve looking for rippling circles of color that represent physical exit points from the Astral to other planes. Some of them are one-way, others are invisible, and most require a tuning fork to attune your frequency to the destination. Only super high-level magic allows for people to simply pass from plane to plane without death or a hop into the Astral.
|The Cosmology of Planescape|
|Inner Planes||Ethereal Plane||Prime Material||Astral Plane||Outer Planes|
|Elemental Planes||Energy Planes||Demiplane of Dread||Plane of Shadow||Plane of Mirrors|
|World Serpent Inn||Tu'narath||Sigil||Demiplanes||Ordial Plane?|