When you’re wearing your Apple Watch, you can use it to automatically unlock your iMac and approve authentication tasks—such as entering passwords, unlocking notes and preferences, and authorizing installations—without having to type a password. These features use strong encryption to provide secure communication between your Apple Watch and iMac.
To use the Auto Unlock and Approve with Apple Watch features:
> System Preferences > Apple ID > Password & Security, then select Set Up Two-Factor Authentication.
Make sure “Disable automatic login” is also selected. (You won’t see this option if you’re using FileVault, but you can still use the “Auto Unlock” and “Approve with Apple Watch” features. For information about FileVault, see Unlock your Mac and approve requests with Apple Watch in the macOS User Guide.