The master password is used for not only locking a note but also backing up a note to your device. More specifically, ‘Auto backup’ doesn’t require the master password, while ‘Manual backup’ and ‘Secured auto backup’ require you to enter the master password.
Once you set the master password, you will be asked to enter it when unlocking or opening a locked note and also when restoring a note backed up with the master password.
Make sure that you don’t forget your master password. Your master password will be stored only on your device. Therefore nobody will be able to access it, not even the ColorNote team. To prevent others from accessing your locked notes, including the case where the device is lost, ColorNote doesn’t provide an option to find the master password. ColorNote places heavy emphasis on security.