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 settings, 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 Settings, click your Apple ID in the sidebar, click Password & Security, then select Set Up Two-Factor Authentication. See the Apple Support article Unlock your Mac and approve requests with Apple Watch in the macOS User Guide.