Dark mode changes the background and many elements of various apps in iOS from light color shades to black and dark grey. When dark mode is enabled, it provides a reverse color scheme intended to improve readability.
Normally text is black and backgrounds are white or light. In a reverse configuration, or with dark mode enabled, text is a lighter color than the background. Dark mode made its debut on the Mac some time ago, bringing the option to darken system menus and the status bar.
Dark mode is expected to launch with iOS 10 on the iPhone 7.