Scio Occultus Res
Mana is the power of magic, the fluid form of Prime, the influence of the Supernal World filtered down into the Mundane World. When the Abyss formed, cutting the Supernal World off from the Mundane World, the Mana in the Mundane World at that time was trapped, and is being endlessly recycled, pulled in and used by mages, and then released back into the world to be gathered and used by others.
A few rare places in the world, called Hallows, still produce Mana that can be gathered and used by the Awakened. Some of these are pieces of the old world that was shattered when Atlantis fell, and still leak the Mana they had stored. Some are sites where Ley Lines meet, creating surges and pools in the flow of Mana across the world. And some still hold a connection to the Supernal World, letting in a tiny trickle of fresh Mana where once they produced a flood.
In some cases, Mana is found in a substantive form, rather than in the energy state. Mages say that it is solid, though it may in fact take the form of a liquid or a gas as well as a solid. They call this substance tass.