Multi-factor authentication is an electronic authentication method in which a user is granted access to a website only after successfully presenting two or more pieces of evidence (or factors) to an authentication mechanism: in this case, your email and mobile device (via SMS or an authenticator app). It protects you from an unknown person trying to access your site.
If enabled in your account, you can set your sites to use MFA in the visitor access settings.
Understanding Multifactor Authentication (MFA)
Setup MFA via SMS
Resetting your visitors MFA device