Although it's convenient to keep extremely private information—such as passwords or ID numbers—in your iPhone's Notes app, you don't want anyone else to be able to access this information. So protect these notes with a password. In Settings, go to Notes, followed by Password. Here, you can set a code or a Touch ID lock. Inside the Notes app, lock a specific note by dragging it to the left in the list, tapping the lock icon, and entering your password.