The Astral Plane is where you go when you engage in astral projection. It is not so much a place, as a state of energetic existence.
The astral plane exists as a sort of shadow to the material world. It is populated with the energetic aspects of material beings, as well as energetic and spirit beings that have no current material / corporeal existence.
Astral Plane in Practice
Humans can only reach the astral plane through altered states of consciousness that allow their spirit or energetic or astral body to travel freely of their material body. Some believe the astral plane is the same as dream space, while others insist they are two different states.
Many traditions consider the astral plane to be composed of several levels, while some postulate multiple astral planes.
At the most basic, Above, where the gods are, Below, where the dead and chthonic spirits reside and here, where Earthly spirits reside- though some will also say that these locations are not part of the Astral Plane.
Existence of Astral Plane
There are much more complicated maps of the Astral Plane and entire books have been written on the subject (See recommendations below). The Theosophists, in particular, have done much astral plane research.
On the Astral Plane, time and space do not exist as they do on the material plane. This allows skilled travelers to move through space and time quite quickly and efficiently to gather information, send messages, and/or effect change on the astral level. This change is generally accomplished by astrally traveling to a location and performing a symbolic action, such as cutting a cord or tying one, placing or removing an object, wiping an area clean, encompassing an area, person or object in an energetic “bubble” or another container, etc.
All beings on the astral plane exist in an energetic or spirit state. This allows communication between beings that may prove difficult when one of them, or even both, is in a material state. In an energetic state, information can be sent and received very quickly, without much explanation.
Very complex concepts, or those that cannot be translated easily, can be transmitted in visual form, (a vision) or some sort of activity can be engaged in to get the message across.
Some people engage in frequent visits to the astral plane in order to lessen or weaken their attachment to the physical/material plane as a form of self-improvement.
Practitioners who visit the astral plane often may wish to build an astral temple.