TL;DR
- Google is adding an additional layer of security when installing an APK or updating an app through an APK.
- A PIN or biometric authentication will now be required in certain cases.
- This will be the case for APKs that Google thinks are malicious or didn’t come from the Play Store.
I hope it is optional. Otherwise it makes sideloading more cumbersome for power users.
If it is a feature of play protect then you should still be able to opt out of it.