Multi-Factoir Authentication (MFA) is an extra layer of security to make sure a person trying to access an account is who they say they are. A password is something you know, but also something that others may know or be able to discover. Without MFA enabled on your Office 365 account, anyone who has knowledge of your username/email address and password would be able to access and misuse your account, but with MFA enabled you are the only person who will receive a notification or code to allow the login. A would be attacker will be unable to access your account without the code or being allowed via the authenticator app. A password is single factor authentication. An additional code or prompt via text or app on your phone is Multi-factor authentication allows for more than your password is required to insure that you are you.