1. What is Azure AD
Official website said:
"Azure Active Directory (Azure AD) is a multi-tenant provided by Microsoft, cloud-based directory and identity management services. Azure AD core directory services, application access management and identity protection into one solution, providing standards-based platform to help developers provide access control for applications based on centralized policy and rules. "
Details can be read
https://docs.azure.cn/en-us/active-directory/fundamentals/active-directory-whatis