Arcane Focus is a Mage talent that reduces the chance that Arcane spells will be resisted. It it useful mostly to high-level mages, especially Arcane specialized raiding mages who need to reduce the amount of resists from their damage spells.
Though the wording might lead one to think otherwise, the chance to hit only applies to Arcane spells.
Arcane spells will have a 3% lower chance to be resisted with 3 points in this talent. Normally, the only enemies that will have a large chance to resist Arcane spells are targets of levels higher than the Mage's, or targets with a high amount of Arcane resistance. In PvP situations, this is very rare, but in raid situations, boss-level targets are considered 3 levels higher and will have a base 17% chance to resist spells.