If you are not familiar with multi-factor authentication or you have not yet enabled the feature on your account, the following resources will provide helpful information:

• What is multi-factor authentication and why is it important?
• How to enable multi-factor authentication for Microsoft Office 365
• What to do if you receive a multi-factor authentication notification when you aren't trying to log in

When multi-factor authentication is enabled on your Office 365 account, and you attempt to log in to a new computer or new web browser for the first time, you will receive a notification to Approve sign in request.  (Note, you may also see this prompt when logging in for the first time following a major windows/browser update or when browsing in private-mode)

At the same time, you will receive a notification on your phone via the Microsoft Authenticator app.

Tap the notification to open Microsoft Authenticator and then tap Approve to verify your login.

You should now be able to access your account like normal.