The While Falling behavior will "watch" a selected object and repeatedly triggers an event while the selected object is in a falling state. A falling state means the selected object has sudden change in vertical velocity, and is continuously changing.
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 create a game where you only want to collect points while your character is falling. You can add to the score while the object is falling.