When Apple introduced the larger-screenedand , it conceded users would need help with using the bigger devices with one hand. Apple's solution is called Reachability. The feature makes it quicker and easier to reach to content at the top of the screen.
Reachability is currently available on the iPhone 6, iPhone 6 Plus,and .
The solution requires you to double-tap the home button. (Notice I didn't say double-press, as that activates the app switcher.) A double-tap, or -touch, will move the top half of content on the screen down to the bottom half of the screen.
You can then tap on a link or button, or scroll through a list or webpage. After tapping on the screen, the content will slide back to its normal position.
In practice, Reachability has been a useful way to quickly tap a button at the top of the screen without having to adjust your grip on the phone or use a second hand. The blank space that takes up the top-half of the screen isn't wasted either. In fact, you can swipe down within that space to.
If you don't like or use Reachability, you can disable the feature on your iPhone. First launch the Settings app, tap General > Accessibility then toggle the Reachability switch to the offposition.
Editors' note, February 1, 2016: This post was originally published on September 22, 2014. It has since been updated to reflect new devices and capabilities.