There was a time where electronics were getting smaller and smaller. But now displays are getting larger and larger. While the extra pixels make for a great viewing experience, it can be hard to reach stuff at the very top of the display.
That’s where Reachability comes into play. If enabled, you can double tap (not click) the home button or swipe down on the home bar (iPhone X) to trigger Reachability.
The displayed content will slide down putting the top contents in the middle of the display, making it easy for your thumb to reach.
To enable (or disable) Reachability go to Settings > General > Accessibility and toggle Reachability on.