Just like a telephone directory (which typically stores names, phone numbers, and addresses) active directory stores information about organizations, users, sites, and any other network objects - and the attributes about those objects - that an organization might wish to track.
Multi-Factor Authentication (MFA) is a security measure that requires a user to both know something (e.g.: a password) and have something (e.g.: a registered device) in order to login to a system. By authenticating their identity with multiple factors, their account is more secure against being compromised.