It is the space between everything. It is where you are when you aren't anywhere else.

The Astral Sea is the space between. Often referred to as "Amongst The Stars". It is full of color and a thick "air", but largely devoid of actual matter.
It is as much a mental and spiritual place as a physical one, and therefore highly subject to shifting perspectives and interpretation.

The Astral Sea is a great multicolored sea, the same above and below, with swirling wisps of many different colors streaking among motes of light like distant stars. Most of the Astral Sea is a vast, empty expanse. Visitors occasionally stumble upon chunks of rock drifting forever in the enigmatic void. Time in the Astral flowed at the same rate on Ignisar but the effects of time were slowed almost to a stop―a thousand years in the Astral felt like only a day to the traveler. Creatures did not go hungry or age while in the Astral. For that reason, its inhabitants needed to return to one of the other realms in order to have children or to reach adulthood.