Since Android™ 5.0, your device is linked to your Google™ account for security measure. Thus, if you loose your device or if it is stolen, your login and your password will be required to pass through the wizard once the device will be restored from the recovery menu.
That means that you must remember the Google™ account you have set in the last wizard if you reset your device from the recovery menu or the Android device manager.
If you have bought the device second-hand, you may have to contact the previous owner to unlock it.
If you wish to resell your device, you have to perform a factory data reset from Android™, do not do it from the recovery menu or the Android device manager.