As part of securing access to your Outlook, Sharepoint, Office, etc, Microsoft will enforce using multi-factor authentication. This means that as well as a username and password (one-factor) you'll need another piece of information to sign in.
Their default is the Microsoft Authenticator app. This installs on your phone and links to your Microsoft 365 account. When prompted, the app will create a 6 digit code which you can then use to log in.
I never needed this before
Indeed, but Microsoft are starting to enforce it across all customers. It's best practice and helps keep us secure.
How do I do it?
At some point, likely when logging into Outlook or Sharepoint online, Microsoft will prompt you to start the setup. You can postpone but you'll have 14 days to complete.
Having clicked 'next' enter your Microsoft 365 password
You'll then be prompted to download the app on your phone. You'll do this using the Play store.
If you already have an authenticator app, eg. Authy, and are confident using it, feel free to opt for this instead and click "...a different method."
Once the app is downloaded on the phone, click 'next'.
On the Authenticator app on the phone you'll be prompted to allow notifications and will have to add an account. Select 'work or school'.
And on the laptop press 'next'
The Authenticator app will offer the option to scan a QR code. To speed up the process, this is our recommended route. Scan the QR code presented by Microsoft (NOT the one in the screenshot below). You must scan the code using the Authenticator app, not using your phone camera.
All being well, the Authenticator app will add the Microsoft account and you'll see the following prompt on the laptop screen.
Microsoft will also offer a phone method of backing up access to your account. To do this, add your work number when prompted.
And ultimately you'll be done.
How do I use it?
Periodically in future, when logging into Office online, you'll be asked to open the Authenticator app and enter the numbers shown, like the following: