That is the "Reachability" feature of iOS and newer phones. The idea is to allow you to be able to access items at the top of the screen when you're using just one hand. It's triggered with a double 'tap' (not press) of the Home button. To turn it off:
`Settings` > `General` > `Accessibility`
Scroll down and look for the `Reachability` setting.
Turn the `Reachability` toggle to `Off`