2FA factor authentication is nothing but authentication is done twice.
For example consider a scenario in login page
when user tries to login to a application with out a 2FA factor .
when user enters valid user name and password he will be directed to a home page of the application.
when user tries to login to a application with 2FA factor .
when user enters valid user name and password. First time the user will be authenticated here and later user will be directed to the next authentication i.e he may enter valid digipass (or) token for the 2nd time.
Once he enters the valid digipass (or) token, User will be directed to Home page of the application.