Action Points(LWR)
In General
At the start of every turn, all XCOM and alien units are assigned 2 action points (APs). This is represented on XCOM soldiers as two blue arrows underneath their health bars.
When a soldier has only 1 AP left, they will have only 1 arrow beneath their health bar and it will be colored yellow.
If a soldier is scampering, they will only be able to move and not using any actions. This is represented by 1 arrow beneath their health bar and it will be colored white.
Using APs
All abilities in the game have the default cost of 2 AP (teal icon). However, units can use abilities that cost 2 AP even if they only have 1 AP left (so the player's APs get below zero). A unit's turn will end if their APs have reached 0 (or less). Thus, a unit can do a half-move (1 AP), and then fire (2 AP), before having their turn ended.
Certain abilities perform an extra check and require the unit to have 2 AP before using them (e.g., sniper rifles or dashing). Other abilities behave differently if the player has 2 AP left as opposed to 1 AP left (e.g., firing rockets or Double Tap).
A full-move (dash) costs 2 AP (blue move) and a half-move costs 1 AP (yellow move). Note that two yellow moves don't let you reach as far as one blue move, but the total cost is the same.
Certain abilities cost only 1 AP (green icon) or 0 AP (purple icon). Just like 2 AP abilities, the 1 AP abilities only end a unit's turn if the unit's APs reach 0.