The Abyss marks a fundamental division in the process of Manifestation. It is the line between the Creator and its Creation; or, phrased another way, between Creation and Manifestation, between the Will and its Expression.
Above the Abyss is only Divinity; creation is Not. That's Briah, the Creative world that is the act of Creation — more particularly, the place where Divinity conceives the desire to create — rather than the creation itself. Formation (Yetzirah), the part we can apprehend, begins below it.
As such, the Abyss is also the division between our everyday consciousness and our Ineffable self, our superconscious. To reach beyond that Veil is to touch our Angel, to bridge the gap between us and our Divine Self.
The Tarot card the Priestess shows us a glimpse of the Abyss. In that card, her arms pull a net upward. Above that net are the swirlings of the Unmanifest, the Supernal triad. Below it is manifest creation in all its variety. The net itself is the Abyss, the boundary between possibility and actuality.