If you take your Apple Watch with you into a plane, then you need to turn the device during the flight in the flight mode. If you do not know how to turn the Apple Watch in airplane mode, then we would like to explain it here in a nutshell:
Return back to the Home Screen or the Watch Face of your Apple Watch. Swipe now from the bottom of the screen upward to display a menu. Swipe now left or right until you see "Settings". Tap this to open it. Within the settings, you can now find the "flight mode", which is marked by a small plane icon.
When you tap this icon, then the flight mode is automatically activated on your Apple Watch. You can recognize this by a "Disconnected" lettering on the top of the screen.
You now know how to activate airplane mode on your Apple Watch and how to deactivate it again.