You’ve been able to unlock your Mac with your Apple Watch for some time now. If your Mac is asleep, and you wake it up, if you’ve activated this feature, the Mac confirms your identity via your Apple Watch and wakes up.
This is an interesting chain of identification. It requires that you have two-factor authentication turned on for your Apple ID, and having authenticated on your iPhone by entering your passcode, your Apple Watch then inherits this authentication (or you can authenticate on the Apple Watch by entering its passcode), and the Mac then accepts this as proof that the watch belongs to you.
To activate this feature, go to System Preferences > Security & Privacy > General and check Use your Apple Watch to unlock apps and your Mac.
This allows you to wake up your Mac, and approve certain secure actions in macOS. For example, if you want to access a secure preference pane – one that shows a padlock at the bottom left of the window – click the padlock then authenticate on your watch by pressing the side button twice (this is the same gesture you use to authenticate for Apple Pay).
Another action where you can use your Apple Watch to authenticate is if you want to delete files in certain folders. For example, to delete an app downloaded via the Mac App Store, you need to authenticate:
If you have a Mac with Touch ID, the Mac defaults to using that option for authentication, but if you have an iMac, which doesn’t offer Touch ID, this can make it a lot easier to perform secure tasks.
Note that this feature is only available to recent Macs, ones that support Continuity and Handoff, not all recent Macs can perform all of these operations. See this Apple support document for more information.