Essence refers to two things in Ars Magica: Raw Essence and reagents.
Main: Raw Essence
Raw Essence is the magical power source, created by a Nexus, that fuels the machines of Ars Magica. It comes in three types: Light, Dark, and Neutral. Raw Essence is primarily used to power essence devices such as Calefactors, Magic Fabricators and Casters.
Essence can also be crystalized into item form by using an Essence Refiner. This is frequently used as a reagent for augmented spellcasting or as a component in creating blocks, items, armors, and used for Spell Research. There are currently 12 types of essence items. Each type of essence exists in opposition to another one. A player can read Affinity Tomes to gain the ability to cast spells of one type at lower cost and without requiring essences, but spells of the opposing essence cost more and require additional essence. You can also throw essence into lava and obtain a lava source.