When you turn on an Android device, it checks the operating system to make sure it's safe to use. This means the code comes from a trusted source and hasn't been changed or corrupted.
If your operating system could have a problem, you can get a warning message that brings you to this page (g.co/ABH).
If you changed the operating system on purpose
You can continue using your device. But you acknowledge that your device might not work properly and your data could be exposed to corruption and security risks.
We recommend restoring your device's operating system. If you have a Pixel or Nexus, learn how to flash your device with the latest factory image.