The Wrist Detection feature plays an important role on the Apple Watch as it allows the device to determine if it is being worn or not.
Wrist Detection needs to be enabled before you can use other features of the Apple Watch, such as the Wrist Raise, heart rate sensor, and passcode.
How To Enable Wrist Detection on the Apple Watch
You can view the status of the Wrist Detection feature by going into the Apple Watch app installed on the companion iPhone. You'll find an option to enable or disable this setting right under My Watch > General > Wrist Detection.