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 either method to open your private notes. You may also be able to use Approve with Apple Watch.
Important considerations when setting the password:
If you enable Touch ID or Face ID for both your device and Notes, anyone with your device’s passcode may be able to access your locked notes.
If you forget your password, the only way to view a locked note is by using Touch ID, Face ID or your Apple Watch (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 two failed attempts. Additionally, you are required to re-enter the password if you update your Touch ID or Face ID settings.
To change the password, you must know the current password.
When you create a password for the notes on one device, all other devices that use the same iCloud account use that password too.
Locked notes are only visible on devices with OS X 10.11.4 or later, iOS 9.3 or later or iPadOS 13 or later; on devices with an earlier OS, you won’t see the notes at all.
To learn more, see Lock your notes.