The caster travels to another player using this spell. This spell is clerical in nature; although many can learn the ways of astral transit, the spell always relies on some deity's intercession. The inventive clerics who created the astral spell intended it as a way to get to people in need. As a result, astral can only be targeted at other players and NOT on creatures (mobiles), even if you're not a cleric and don't intend to help anyone.
This is one of the only transport spells available to Black Circle Initiates. However it has many limitations such as not being able to astral to a player in an infirmary flagged room (as well as cursed/no-recall rooms of course).