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:
Sign in to your Mac and Apple Watch with the same Apple ID.
Make sure your Apple Watch is unlocked and running watchOS 3 or later to automatically unlock your Mac; approving authentication requests requires watchOS 6.
Turn on two-factor authentication (see below).