Azure Active Directory (Azure AD) is Microsoft’s cloud-based identity and access management service, which helps your employees sign in and access resources in:
External resources, such as Microsoft Office 365, the Azure portal, and thousands of other SaaS applications.
Internal resources, such as apps on your corporate network and intranet, along with any cloud apps developed by your own organization.
As an app developer, you can use Azure AD as a standards-based approach for adding single sign-on (SSO) to your app, allowing it to work with a user's pre-existing credentials.
As an IT admin, you can use Azure AD to control access to your apps and your app resources, based on your business requirements. For example, you can use Azure AD to require multi-factor authentication when accessing important organizational resources.
For more info refer https://docs.microsoft.com/en-us/azure/active-directory/fundamentals/active-directory-whatis