When Background App Refresh is on, apps can exchange data between the iPhone and Apple Watch, so they always have current data when needed.
This can affect battery life, of course, so you can turn the feature off entirely, or select specific apps.
1. Start the Watch app on your iPhone.
2. On the "My Watch" tab, tap "General."
3. Tap "Background App Refresh."
4. On the Background App Fresh page, you can disable the feature entirely by swiping the button at the top of the screen to the left. Or, you can enable and disable specific apps so only the apps most important to you get refreshed automatically.