Two-factor authentication is an extra layer of security for your Apple ID designed to ensure that you're the only person who can access your account, even if someone knows your password. Here are the steps to take to enable it on macOS Sierra.
- Go to Apple () menu > System Preferences > iCloud > Account Details.
- Click Security.
- Click Turn on Two-Factor Authentication.
Some Apple IDs created in iOS 10.3 or macOS 10.12.4 and later are protected with two-factor authentication by default. In this case, you see that two-factor authentication is already turned on.
If you already use two-step verification and want to upgrade, turn it off, then turn on two-factor authentication. To turn it off:
- Sign in to your Apple ID account page.
- In the Security section, click Edit.
- Click Turn Off Two-Step Verification. Click again to confirm.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)