The user interface of your FRITZ!Box is either protected with a unique password in the factory settings or you must assign your own password the first time you open the user interface. If the FRITZ!Box user interface is already protected with a unique password in the factory settings, the password is located on the bottom of the FRITZ!Box.

With the FRITZ!Box password, you can access the user interface on all of the devices connected to the FRITZ!Box. To connect a device via Wi-Fi, you need the network key that is also on the bottom of the device. You can then configure settings in the user interface and change the Wi-Fi network key or default password. 

If you changed the FRITZ!Box password and then forgot it, there is no way to find out what it is. It is therefore best to configure the "forgot password" push service right away, so you can simply have the password sent to you by email. If you forget the password and have not configured the push service, you cannot open the user interface again until you have restored the factory settings.