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Factory Reset Protection (FRP) is a security method that was designed to make sure someone cant just wipe and factory reset your phone if youve lost it or it was stolen. Starting with Android Lollipop, FRP is "standard" in vanilla Android, and most companies making our phones have implemented it in their own models. Its a good thing — it makes a stolen phone harder to use, which makes it less appealing to thieves, and anything that can protect our data on a phone weve lost is welcome.
      The problem is that people are selling or trading or even giving away phones with FRP enabled and this makes things difficult for the next user.

How it works explains why. If you reset a phone with FRP enabled, you have to provide the user name and password for the last Google account that was registered with the device. There are random work-arounds on the Internet, but they tend to get patched almost as soon as they are discovered. Youll pretty much need to know the login details for the last account to use the phone before you can do anything with it if FRP was enabled before you reset it.