Google Factory Reset Protection is a security procedure that hinder anyone form cleaning all data on the Phone. This is viable solution in case of theft or in some cases pranks. But what if someone intentionally sell their cell phone and wants to completely remove its private data and make it usable again.
Google Factory Reset Protecting (FRP) was introduced in Android Lollipop and became a standard in Android Vanilla. Almost all mobile vendors implement FRP as per their need. It is activated when you set-up a google account on your Android phone.
When the phone is retested to factory settings either from settings or via boot menu. After resetting and wipe is done the google ask you to enter credentials of previous user.
Procedure to remove this screen varies from vendor to vendor but in nutshell you need to access Settings and from their remove the google account in question and the cell phone will be reset completely. Following videos show how to Bypass Google Factory Reset Protection for different phones.