What Is Microsoft Authenticator? Why am I prompted to download and install Microsoft Authenticator?

Microsoft Authenticator is a cellphone app that provides an additional level of security to your Azure AD work or school account or your Microsoft account. With the Microsoft Authenticator app, users can authenticate in a passwordless way during sign-in, or as an additional verification option during self-service password reset (SSPR) or Azure AD Multi-Factor Authentication events.

You will be prompted to download and install Microsoft Authenticator, if your organization has turned on the "Verification code from mobile app or hardware token" as part of the "multi-factor authentication" settings.

Here is an example:

1. Login to your Microsoft business account with a browser at https://www.office.com. You see the following warning message.

Help us protect your account 

Microsoft has enabled Security Defaults to keep your 
account secure. 

[Learn more about the benefits of Security Defaults]
[Skip for now (14 days until this is required)]

[Next]

2. Click "Next". You see the following "Microsoft Authenticator" prompt:

Keep your account secure

Your organization requires you to set up the following methods 
of proving who you are:

Microsoft Authenticator
  Start by getting the app 
  On your phone, install the Microsoft Authenticator app.
  Download now. After you install the Microsoft Authenticator
  app on your phone, choose "Next".

Note this you will see this warning and download prompt, even you have disabled the "multi-factor authentication" feature on your account.

To remove this this warning and download prompt, your admin must go to the admin center and uncheck the checkbox on "Verification code from mobile app or hardware token" as shown in the previous tutorial.

⇒ Using Microsoft Office Tools Offline

⇐ Change Multi Factor Authentication Options

⇑ Microsoft 365 for Business Admin

⇑⇑ Microsoft 365 - Frequently Asked Questions