Quintessence is a quantity and thing in the White Wolf storyteller game Mage: The Ascension, representing the basic building block of the universe.
A given mage may have a Quintessence score anywhere between 0 and 20. This is tracked on the same scale that paradox is, save going the other way. If your quintessence score should approach your paradox score, the paradox wins out. Essentially, you can never have more quintessence than 20-paradox.
A mage can hold as much quintessence as his avatar rating. He may also have on his person tass*, or solid quintessence. If the mage posesses the sphere Prime 1 or greater, he may hold more than his avatar rating in quintessence.
To regain quintessence, the mage can refuel his avatar by resting at a Node** (unrelated to E2 nodes). A Node is a natural wellspring of quintessence in the tapestry, caused either by a mage with Prime 5, strange geography, or apt timings (better known in this case as a Junction). With Prime 5, though, the mage can draw quintessence out of the natural wellspring of the tapestry.
A mage can use quintessence to strengthen his own magical effects, giving him a difficulty break on his effects. A mage may only spend an amount of quintessence equal to his avatar rating per turn, reducing the difficulty to a maximum of -3 from the original. Each point spent reduces the difficulty by one.
Quintessence can also be used to strengthen reality around the mage, acting as a counterspell of sorts, increasing the difficulty of another mage's effects. Unfortunately, most players don't use this fact, much to the chagrin of storytellers.
All in all, Quintessence is a nice addition to Mage: the Ascension, allowing for some creativity and a chance to break limits of a very troublesome system for most people to play in.
* - Tass is quintessence pooled into an object, often as result of being exposed to a node.
** - A Node is a natural physical wellspring of quintessence.