You have 3 actions and 1 reaction per round. You regain all actions at the start of your turn. Actions can be any of the following sorts, or actions specifically listed elsewhere.
A reaction is a special kind of action that is used when it’s not your turn. A reaction typically resolves before the action that triggers it.
All characters have access to “free actions”, actions that take no time at all. This might be saying a single line of dialogue, dropping something, etc. A GM is the final arbiter of how many free actions you can take in a round.
Some kinds of actions take all your effort on a given turn. These are called “full round actions” and they take all 3 of your actions on your turn. Taking a full round action precludes you from using things like “additional actions” as well: it takes up all your actions. You can still take free actions however.
Some classes and abilities give you an extra action on your turn to do a specific thing. (Example: A primary fighter can spend a Hero Dice to make an extra attack action.) You can only ever get 1 additional action per turn.