From the White Wolf Wiki, the most complete White Wolf reference

Prime is the Sphere of magic that governs the raw energy of reality. A Mage who uses Prime can sense magic of other Magi, use the power of the universe to create Effects out of nothingness and can store Quintessence past their Avatar background rating. It is the study of Quintessence and it is regarded by all the Traditions as a valuable or holy power. With Prime, Magi can create, destroy or reshape the universal energies.

"While other Spheres influence the actual stuff of reality, Prime controls the power from which they all spring" (Mage: The Ascension, 179). With Prime, a Mage can enchant objects and living creatures with magical energy, fight back the effects of Paradox, use CounterMagic and make the changes they exact on the world permanent.

Mage: The Ascension Spheres

Correspondence · Dimensional Science · Entropy · Forces · Life · Matter · Mind · Prime · Spirit · Time