Understanding Azure Account, Subscription and Directory.

I would probably start with the following links:

What is Azure Active Directory:

https://docs.microsoft.com/en-us/azure/active-directory/active-directory-whatis

The relationship between AAD and subscriptions:

https://docs.microsoft.com/en-us/azure/active-directory/active-directory-how-subscriptions-associated-directory

Managing resource groups with AAD:

https://docs.microsoft.com/en-us/azure/active-directory/active-directory-manage-groups

From each of the links above, there are multiple other links to a lot of content that will explain all these differnet components and their relationships.

Aside from the "docs" website, I also have found that the Microsoft Virtual Academy website is a great source of information:

https://mva.microsoft.com/

Cheers,

Stephane