what is the passcode?

A passcode is something you can create going to Settings > General > Passcode Lock that will require entering a 4-digit number to unlock the iPhone. Users create passcodes to restrict access to their iPhones. However, if you forget the passcode, you will not be able to unlock the iPhone without restoring to factory condition and then restoring your backed up content. So, if you use a passcode it is important to maintain a current backup in case you end up disabling your iPhone.