That is a setting in Settings>General>Accessibility in the 3D Touch section at the very bottom called "Reachability". When you have that turned on, and you double-tap (not press, but just lightly tap) the Home button, it brings the top of the screen into reach.
This feature is used when you are holding the device with one hand and trying to type or select something that is located at the top of the screen, which is difficult on a larger phone unless you have really long thumbs.
Cheers,
GB