The Digital Crown on the Apple Watch is Apple's answer to the crown found on mechanical watches. On traditional watches, the crown is used to set the date and time on a wristwatch. There are various ways you can use it in watchOS 3.
Double-click it to return to the last used app. Single-click it to return to the home screen. Press and hold it to use Siri.
You can turn the Digital Crown to use Time Travel, zoom, scroll, or adjust what's on the screen. On the Apple Watch Series 2, turn it to gradually wake the display and slowly increase the brightness. Also, on Apple Watch Series 2, turn the Digital Crown to unlock the screen during a swimming workout.