This is an information only field on the User Card.  When using NAV or Business Central OnPremise the middle tier server can be configured for Office 365 Single Sign-on experience.  The users Office 365 email account is then added as the "Authentication Email" and used for confirming the user signed on is a valid user.

This field indicates the user’s status for Office 365 authentication. When you start to create a user, the status is Disabled. After you specify an authentication email address for the user, the status changes to Inactive. After the user logs on successfully, the status changes to Active

For more information, see How to: Create Microsoft Dynamics NAV Users also  Authentication Email.

Note: Office 365 Authentication is automatically managed when using Microsoft Dynamics 365 Business Central Online (SaaS)