The Started Falling behavior will "watch" a selected object and trigger an event as soon as the selected object has entered a falling state. A falling state means the selected object has sudden change in vertical velocity.
This is the object that will trigger the event when it falls. 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.
If you wanted to open a parachute when your object starts to fall.