Apps form Google Play are always verified by Google to protect you and your device from harm. By default, your device doesn’t allow the installation of apps from sources other than Google Play.
If you choose to allow the installation of apps from unknown sources and install apps from sources other than Google Play, it’s more likely that apps will be installed that could harm you or your device. If you allow the installation of apps from unknown sources, the Verify Apps feature protects you when installing apps outside of Google Play by continually checking your device to make sure that all apps installed are behaving in a safe manner, even after installation.
For more information or to change the default behaviour, visit Protect against harmful apps.