It just where Google has noticed a ‘gap’, ie the user probably stopped and visited something. But Google is unsure. ie possibly did an unknown activity.

Why does it say missing activity on my timeline?

This could be due to motion, turning the device on and off, or simply a poor signal on the WiFi / 4g / GPS connections it used to determine its location.

How do I know if my Google timeline has been edited?

Check the status of your edits
On your Android phone or tablet, open the Google Maps app. and make sure you’re signed in to your Google account.Tap Contribute .At the bottom, tap See your edits.

Go to Settings> Scroll down to System, Select Automatic Date & Time. Switch between Accounts and see default Account, Confirm Default Account is using in Google maps.