Quick answer: Open the note in Notes, tap the More button (three dots) and tap Lock. Choose your iPhone passcode or a separate password, and turn on Face ID to unlock it. The note's title stays visible, so keep sensitive details out of the first line.
Most of what's on your phone is fine for anyone glancing over your shoulder to see. A few things aren't: an ID number, a door code, a medical note, a private photo. iPhone has a way to lock each of these, but they live in different places and protect different things.
Lock a note in Apple Notes
Open the note, tap the More button (the circle with three dots), and tap Lock. The first time, iPhone asks whether to use your iPhone passcode or a separate notes password, and lets you turn on Face ID so unlocking takes a glance.
One detail catches people out: a locked note's title stays visible in the notes list. Only the body is hidden. So never put the sensitive part in the first line. "Bank info" is a fine title; the account number itself is not. Shared notes can't be locked, so keep private notes unshared.
Hide private photos behind Face ID
In Photos, select the photo, tap the More button, and choose Hide. It moves to the Hidden album, which on current versions of iOS is locked behind Face ID by default (there's a "Use Face ID" switch in the Photos section of Settings if yours isn't). The photo disappears from your main library and from widgets and memories.
Passwords belong in Passwords, not Notes
It's tempting to keep logins in a locked note. Don't. The built-in Passwords app is designed for exactly this: it fills them in for you, syncs securely across your Apple devices, and warns you about reused or leaked passwords. A locked note does none of that. Wi-Fi passwords are a small exception worth knowing about; here's how to find and keep those.
Lock individual saved items in Snugit
Plenty of private things aren't notes at all. It's a PDF of a lease, a screenshot of an insurance card, or a link to a medical portal. In Snugit you can lock any single saved item so it needs Face ID (or Touch ID) to open, while everything else stays one tap away. It's included on every plan, including free, because privacy shouldn't be an upsell.
What locking doesn't do
A lock keeps out someone holding your unlocked phone. It doesn't protect against someone who knows your passcode, since that can unlock most of these too. It also isn't a backup. If losing the information would hurt, make sure a copy lives somewhere else as well. And screenshots of locked content are ordinary photos. If you screenshot a locked note, that image sits unprotected in your camera roll.
Download Snugit to keep private files, links and notes in one place, each locked with Face ID when it needs to be.