The Became Active behavior will trigger an event once immediately when your object enters an active state. An active state implies an object is moving, rolling, or colliding.
This is the object you want to check when it becomes active. 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.
In a physics based scene, you can make sure one objects behaviors will only execute if another object is active.