When you log in to your computer using your username and password that is considered a single form of authentication. Multifactor authentication requires you to provide at least a second form of authentication to verify your identity.
Unless you have already registered your information in the My Profile portal, after your account is enabled for multifactor authentication, you will be prompted to register the next time you login. It will look like this:
Open the Microsoft Authenticator app on your phone, select to allow notifications (if prompted), select Add account from the Customize and control icon on the upper-right, and then select Work or school account.
Note: The first time you set up the Microsoft Authenticator app, you might receive a prompt asking whether to allow the app to access your camera (iOS) or to allow the app to take pictures and record video (Android). You must select Allow so the authenticator app can access your camera to take a picture of the QR code in the next step. If you don't allow the camera, you can still set up the authenticator app, but you'll need to add the code information manually.
Scan the provided code with the Microsoft Authenticator app QR code reader, which appeared on your mobile device after you created your work or school account in the previous step.
The authenticator app should successfully add your work or school account without requiring any additional information from you. However, if the QR code reader can't read the code, you can select Can't scan the QR code and manually enter the code and URL into the Microsoft Authenticator app.
Select Next on the Scan the QR code page on your computer. A notification is sent to the Microsoft Authenticator app on your mobile device, to test your account.
However, if you prefer, you can select I want to set up a different method, circled in the image below.