Action points can be used in the following manner:
Augment any d20 roll before success or failure of the roll is known. This does not apply to the confirmation of Critical Hits.
Spend one action point to stabilize yourself if you are dying..
Spend two action points to negate the instant death effect of a double critical hit, you stabilize at 0 hit points.
Artificers can spend one action point to quicken an infusion.
You can spend 2 action points to gain another use of a class feature (smite evil, shift, etc).