(Passive) Hero is refunded an action point for each melee kill.
A hero with Thornfang receives a bonus action point each time they kill an enemy with a melee weapon. The action point is gained even if the melee attack would otherwise end the hero's turn, allowing the hero to chain together multiple kills in a single turn. The bonus action point can be spent on any valid ability (not just attacks).
Note that an attack that ends turn will use all remaining action points, so even if the hero had 2 AP remaining when making the melee attack, the bonus from Thornfang brings them back up from zero to one.