Ability power (AP) is a hero stat that increases the effectiveness of most abilities. It stacks additively.
Effects that benefit from ability power receive a bonus based on a certain percentage of a hero's current ability power. For example, if a skill deals 100 + [0.2 AP or 20% AP] damage and a hero has 50 ability power, the skill's damage will increase by 10. In-game, skill descriptions do not display the percentage, but do display the resulting bonus; the previous example would display as 100 + (10).