Hi @Pratyusha Menon ,
The terms "tenant" and "directory" are for the most part interchangeable and are used that way in Azure documentation.
A tenant is an instance of an Azure Active Directory. The tenant is an account in Azure that comes with a subdomain and an associated Azure Active Directory. In order to use an Azure Active Directory you need to become a tenant within the system. So a tenant is basically securing a .onmicrosoft.com subdomain. At that point you would have one account registered in your Azure AD.
As you mentioned, a subscription is associated to a single Azure Active Directory, but you can add multiple subscriptions to the same directory. One reason you might do this is to separate the finances and administration within a company. For example, a company might have a single org-wide tenant, but different Azure subscriptions for each department. That way the company can track how much money each department is spending on resources.
Another reason for doing this would be to divide subscriptions for different development purposes such as having a sandbox environment, staging environment, and production environment that each have different subscriptions attached.
Hope this helps and let me know if you have further questions!