When enabled, Dark Mode will switch from a predominately white or light colored interface to a nearly black interface. The benefits here are two-fold. First, according to Google, dark mode saves battery life. The other reason is that dark mode is easier on your eyes when viewing screens in dark or dim environments.
iOS 13 and iPadOS 13's new Dark Mode will work across all system apps, with Apple providing developers with the tools to fully support dark mode in their apps.
If you're curious about how Dark Mode looks in iOS 13 in various apps, Apple was kind enough to post some screenshots.
Here's Dark Mode in Messages:
As you can see, every aspect of the interface is now darker. There will undoubtedly be an adjustment period if you decide to use Dark Mode, which, by the way, can be turned on and off at any time, or you can set it to automatically turn on and off based on the time of day. As for that weird line across the keyboard in the screenshot above, that's iOS 13's new QuickPath gesture-based typing feature.
Here's a look at Dark Mode in Maps, the homescreen, and the Notes app:
The changes are most apparent in Maps and Notes, however, if you look at the app dock in the homescreen image, you'll notice that it, too, is darker.
And just because Apple included them, here's Dark Mode in Mail, Photos, Reminders, and the Shortcuts app:
iOS 13 will be released this fall and is currently available to developers as a beta. Apple has said they will run a public beta program for iOS 13 starting in July.