Ability Power is a stat found on all equipment. Ability power effects the spell power of a specific item. For example, ability power on a bag effects how much a person can carry. Ability power on a weapon effects how much damage is done. Ability power on a Mount effects the armor and carrying capacity, but not speed. Ability Power does not stack between items, it only effects the specific item.
Ability power was explained by Empa during the summer 2015 alpha.