This is the core skill for all spellcasting. It helps to succeed in essentially every form of magic. Spells also require proficiency in the cast skill for their cast type for reliable success however.
The in game help reads:
Skill duration: random. Type of skill : spell casting skill (arcane skill). Affecting stats: None. It uses no endurance points. This skill is the basis upon which a spell caster draws all of his ability. Imbued in this skill is the general learning and knowledge of magic and its use. Even the most basic of incantations require some knowledge of how to correctly speak the verbal components, and execute the proper accompanying physical actions to work. Those who aren't well schooled in this learning can sometimes get lucky and successfully employ their power some of the time, but who wants to trust luck?