Actions
You primarily affect the world around you by using actions, which produce effects. Actions are most closely measured and restricted during the encounter mode of play, but even when it isn’t important for you to keep strict track of actions, they remain the way in which you interact with the game world. There are three types of actions: single actions, activities and free actions.
Single actions can be completed in a very short time. They’re self-contained, and their effects are generated within the span of that single action. During an encounter, you get 3 actions at the beginning of your turn, which you can use as described here.
Activities usually take longer and require using multiple actions, which must be spent in succession. Stride is a single action, but Sudden Charge is an activity in which you use both the Stride and Strike actions to generate its effect.
Free actions don’t cost you any of your actions per turn. A free action with no trigger follows the same rules as a single action (except the action cost), and a free action with a trigger can be used anytime that its trigger is met.