Google introduced factory reset protection in Android 5.1 to prevent a stolen device from being used. When FRP is active, you're supposed to be required to log in with an account that was previously on the device, but RootJunky has made it his mission to find workarounds for FRP. In fact, he found a fantastically complicated one for the Android N developer preview.
The video above shows what it takes to bypass factory reset protection on the Android N dev preview. It's no simple feat, but it does appear to work. The goal of these workarounds is to reach the settings where the device can be factory reset again, thus removing the FRP login requirement. Starting with the keyboard settings, it takes around 20 steps to reach the settings on Android N, depending on how you count. It's quite a ride.
Android N is a developer preview, and there are plenty of warnings that things like this could happen. The FRP bypass is just another reason you shouldn't use the dev preview on your main device. Now that the workaround has been detailed, I'm sure Google will fix it in a future dev update, or at least in the final build.