About setting the password
Locking your notes keeps them private from others who might use your Mac, iOS device, or iPadOS device. Notes uses a password to lock the notes you choose—both notes in iCloud accounts and notes stored on your device. If you can access your notes from a device that supports Touch ID or Face ID, you can use your fingerprint or face instead of the password for most actions. You may also be able to use Approve with Apple Watch. (To change the password, you must know the current password.)
Important: If you forget your password, the only way to view a locked note is by using Touch ID, an approved Apple Watch, or Face ID (if you have a device that supports one of those features and you enabled it before you forgot your password). For that reason, it’s strongly recommended that you add a hint when you create your password. Notes shows the hint after three failed attempts.
When you create a password for the notes on one device, all other devices that use the same iCloud account use that password, too.
Important: Locked notes are only visible on devices with iOS 9.3 or later, or OS X 10.11.4 or later; on devices with an earlier OS, you won’t see the notes at all.
To learn more, see Lock your notes.