Using Sierra’s Auto Unlock
You can unlock a Mac when your Apple Watch running watchOS 3 is nearby. Assuming your Watch has a passcode set and two-factor authentication is enabled for your Apple ID, you can turn on Auto Unlock in System Preferences’ Security & Privacy pane. Try to unlock your Mac; it’ll say it’s doing so with your Watch. If the Watch is absent, you’ll get the standard password screen. Sadly, there’s no automatic locking based on proximity, but MacID can do that (see below).