How your character acts is broken down into actions, reactions, and activities. These are defined with a standardized format which lists the action/reaction/activity’s action cost, name, and other information.
In Combat Mode, each character gets three actions on their turn, plus one reaction. These are spent as you see fit, then they reset at the beginning of your next turn. In Exploration Mode and in Downtime Mode, actions and reactions generally aren’t tracked, though activities could take minutes, hours, or even days.
An “action” or “single action” is anything which takes exactly one action to accomplish. Common examples include Strike (hitting an enemy with a weapon or unarmed attack) and Stride (move your speed in combat).
A “free action” doesn’t consume any of your three actions. Most free actions are taken before or after another triggering action, though some free actions like talking don’t have such restrictions.
A “reaction” is an action taken in response to a triggering event. Common reactions include the Shield Block feat and Attack of Opportunity (5e players should note that it works differently). Reactions can be taken on any creature’s turn, including your own.
An “activity” is any other thing that you can do which isn’t covered by the above action types. Some activities take two or three actions, while some can take minutes, hours, or days. Common activities include Cast a Spell and Treat Wounds.