The Became Idle behavior will trigger an event once immediately when your object enters an inactive state. An inactive state implies an object is idle and no longer moving.
This is the object you want to check when it becomes idle. By default the current object is selected, but you can tap the graphic and choose any other object in your scene.
This outputs the Object ID.
You can use this for an AI state. By checking the inactive state of your player, the AI can stop or no longer patrol. When the object is inactive, disable behaviors, or run other necessary logic.