Azure Active Directory (AAD) is a new capability to allow you to have cloud based directory. In AX2012 R3 there was a capability added to allow claims users to be authenticated through Azure Control Services (ACS). AAD can be used through ACS so this gives you the ability to manage users that need to access the AX Enterprise Portal to be authenticated with AAD. This video gives a walk through of the basic concepts.
I did a follow-up post you can find here that covers more detail