Action points give characters the means to affect game play in significant ways, by improving important rolls or unlocking special abilities. Each character has a limited number of action points, and once an action point is spent, it is gone for good.
Acquiring Action Points
Characters do not naturally gain action points. In order to gain or use action points, a character must possess the Heroic Spirit feat. (In some games, the DM may grant this as a bonus feat to all player characters.)
A character with the Heroic Spirit feat gains 5 action points each time a new character level is gained. There is no limit to the number of action points a character may possess at one time.
Some classes or feats may increase the number of action points gained with each level or may grant bonus action points.
Using Action Points
You may only spend action points once per round, though you might be able to spend more than 1 action point at that time.
You can spend action points on any of the following actions.
Modify a d20 Roll
When you make a d20 roll—such as an attack, save, or skill check—you can spend 1 action point to add 1d6 to that roll. This d6 is called an Action Die. You can declare the use of 1 action point to modify a d20 roll after the roll is made, but only before the GM reveals the result of that roll.
You can’t use an action point to alter the result of a d20 roll when you are taking 10 or taking 20.
Depending on character level, you might be able to roll more than one action die when you spend an action point. If so, apply the highest result and disregard the other rolls. At 8th level, you roll 2d6 action dice. At 15th level, you roll 3d6 action dice.
For example, a 15th-level character gets to roll 3d6 and take the best result of the three. So, if he rolled a 1, 2, and 4, he would apply the 4 to his d20 roll.
Activate a Class Feature
Some class features—such as a barbarian’s rage, or a cleric’s channel divinity—have a limited number of uses per day. You may spend 1 action point to activate one of these abilities that you possess without expending one of your daily uses of that ability. You may activate one of these abilities even if you have no more daily uses remaining.
Make Additional Attack
During any round in which you make a full attack, you may spend 1 action point to make one extra attack at your highest base attack bonus.
Stabilize
If your character is dying, you may spend 1 action point on your turn to immediately become stable at your current hit point total.
Use a Heroic Feat
Most Heroic feats can be activated by spending 1 or more action points.
Variant Action Points
There are several variants to action points presented in the articles linked below.