Action points allow creatures to take more actions than normal. Adventurers each start with 1 action point, and some monsters have action points, as noted in their stat blocks. Typically an adventurer spends an action point to pull off an extra attack, to move farther than normal, to take the second wind action in the same turn that he or she attacks, and so on.
Spend an Action Point
Action: Free action. A creature must have an action point in order to take this action. Also, the creature can take this action only during its turn, and never during a surprise round.
Gain an Extra Action: The creature gains an extra action to use during its current turn. The action can be a standard, a move, or a minor action. Some creatures have special abilities that trigger when they spend action points, and some have the option of gaining a benefit other than an extra action when they spend their points.
Once per Encounter or Round: An adventurer can spend an action point only once per encounter. In contrast, a monster that has action points can spend more than 1 action point during an encounter, but only 1 per round.
Gone When Spent: An action point is gone when spent, regardless of the success of the action taken.
Published in Player's Handbook, page(s) 286, Monster Manual 2, page(s) 6, Rules Compendium, page(s) 235.