The app will run background testing on all Android devices if this is enabled on your phone. This means that the suite of tests will run approximately once a day, whether you're using the app or not. The app checks the device environment, including whether your phone is connected to wi-fi, if any other apps are running, whether the device has sufficient battery power, or whether the device is connected to a charger. If all of the constraints are met then the android operating system will allow the app to run a test.
iOS devices don't run background tests, it's not allowed as part of the operating system, so you'll only see this on Android devices!