Every action has a cost in Action Points (AP).

AP starts at 0. When an action is take, the cost of the action (e.g. walking = 100 AP, running = 50 AP) is subtracted from the total entity AP. Now the entity is sitting at -100 AP. Each round AP is increased by an amount calculated based on the entity's "speed" attribute. e.g if entity speed = 25, it will take 4 rounds for the entity AP to get back to 0, at which point the entity can take another action.