Action Points (APs) are points gained in the process of increasing in level. You can the spend those points into enhancements. However, you must meet the progression and any prerequisite there is, otherwise you won't be able to pick the desired enhancement.
Enhancements cost from one to six Action Points. For this reason, you do not lose your action points when leveling up. As of Module 3.3, you can have as many enhancements as you want. Upgrades of the same enhancement replace previous versions. However, you need the previous version of an enhancement to make its highest version available to your character.
You receive an action point at ranks 1, 2, 3 and 4, for a total of four action points per level. At the level cap (20), there are no ranks. You instantly gain the last four Action Points for a final total of 80.