An action is a game command or behavior that can be triggered once or continuously depending on which Action you are using. Actions are triggerable bits of logic that you can use to build logic into your game. An action can destroy the entity that it is called on, or change the direction of an entity.
GameBuilder Studio comes with a set of pre-defined actions that can be added to a Rules component or a State Logic component and are triggered based on different conditions in the game. You can create custom actions by writing your own plugin.