Mana is the fluid form of the Materia Prima, otherwise called Prime, Supernal energy filtered down into the Fallen World. Some of it is residual energy left trapped when the Abyss formed, recycled over the millennia into a thousand forms, but some of it is fresh, brought into the world by a mysterious form of grace from on high. In a Hallow, it can also be crystallized into a material form called Tass.
Mana itself is normally invisible and intangible, undetectable to those without Mage Sight. Mages perceive Mana in many different ways. It is most often described as a fiery, fluid energy, either pure white or prismatic, containing all the colors of the spectrum. Some mages perceive Mana as another sort of power or force, in accordance with their own Path. The particular Arcanum a mage uses for his Mage Sight often colors the experience.
With the use of their Gnosis, mages are able to harness the power of Mana for various purposes, like empowering their spells, use foreign Arcana, use sympathetic spells or to heal their wounds. There are also various ways of regaining Mana, most involving a Hallow. Other ways involve scouring their own body or even a blood sacrifice.