Locked lists

Locking a list means it cannot be opened without a PIN or a biometric check. It is for lists you would rather someone could not read if they picked up your unlocked phone or sat down at your browser.

Setting a lock

Turn on the lock for a list, then set an unlock method: a PIN of at least four digits, or a biometric such as Touch ID or Face ID where the device supports it.

Opening the list then asks for that unlock.

Locks are per device

This is the part worth understanding. The unlock method is set up on the device you set it up on. Opening the same list on another device means setting up an unlock method there too, after confirming your account.

That is deliberate: it means a PIN never leaves the device it was set on, and it is why a lock protects against someone using your device rather than against someone getting into your account.

Locking is not encryption

A locked list is stored the same way as any other list. The lock controls whether the app will show it to you on that device.

If you want the contents themselves encrypted, that is Vault protection, which is a separate feature and can be used with or without a lock.

If you forget the PIN

You can set up a new unlock method on that device after confirming your account. The list is not lost, because the lock is an access control rather than a key to the data.