It is an online function from Google where Google checks with elevated privileges if your system isn't rooted, that no custom firmware is installed and the boot

It is an online function from Google where Google checks with elevated privileges if your system isn't rooted, that no custom firmware is installed and the bootloader is locked. Some banking apps require this service to "certify" your device too. Devices that fail this check cannot install some apps via the play store (netflix is a prominent example, don't know if that is still true though). But also some banking apps refuse to launch when the check is failed. Of course it doesn't really say anything about the safety of your device. It is a check that your device really doesn't belong to you and you cannot do everything with it.