Reachability is a feature on iPhones that allows users to lower the top half of the screen to make it more easily accessible with one hand. It works by virtually lowering the upper portion of the screen so that users can reach it. This feature is particularly useful for larger iPhone models, which can be difficult to use with one hand. To enable Reachability, users need to have an iPhone 6 or later and turn on the feature in the "Accessibility" settings. Once enabled, users can activate Reachability by swiping down from the bottom edge of the screen or double-tapping the home button.