Learn how to setup a multi-factor authentication (MFA) or 2-Factor Authentication for Office 365 users. In Office 365 world, they called it as MFA.
You have two steps to make it happen.
Set up multi-factor authentication for Office 365 users – Office 365
And the second steps is for your user to to set up their second sign-in method for Office 365.
Step-by-step procedures. Set up 2-step verification for Office 365
By setting up 2-step verification, you add an extra layer of security to your Office 365 account. You sign in with your password (step 1) and a code sent to your phone (step 2).
Step-by-step procedures. Create an App Password for Office 365. The App Password is a One-Time Password (OTP) that you can create and use for MS Outlook, iPhone / Android email, and Skype for Business.
Please Note: The App Password is a one time password and never expires. Though you can delete and replace it if you want to update your app password.