As part of the cosmology of existence, the astral plane connects with all the other planes and acts as the main passageway between planes. A connection with this plane is the functioning part of Bags of Holding and other like items as well as transportation spells, such as Dimension Door or Teleport.
The plane itself is largely non-physical and appears as a great vista of light and darkness with intermittent flashes of colour and grey debris (which is created by destructive magical forces unleashed on the Prime Material Plane). There are no distancesper se; the familiarity with items or places within the plane determines the amount of time it takes to reach those places.
Likewise, the intelligence of the traveller determines the speed of travel. An unintelligent creature, like a golem or a fungus, travels very slowly. A smart creature, regardless of their size, moves very quickly.
Time moves at its own pace within the astral plane. A person who projects themselves into the plane and spends several hours there would hardly notice any elapsed time between their departure and return.
The astral plane has frequent visitors or travellers. It also has races and individuals who reside in the plane itself. The Githyanki, for example, use the astral plane as their native dwelling place and have constructed cities and fortified places within the plane. Given the power of those who enter this plane or who use it to travel from plane to plane, this is a very dangerous place.