Double-tap wake is a very convenient feature. When enabled, you can wake up the screen by double-tapping the screen. Here are the steps to enable it:
Step 1: Swipe up on the watch home screen and select Settings.
Step 2: Scroll down and click on Display.
Step 3: Select the Double-tap to wake option and enable it.
Now, when the screen will be off, you can double-tap on it to wake up the screen.
Source : wearablestouse.com