The billing and access-control unit in Azure. It contains resource groups and resources. An organization can have multiple subscriptions to separate environments (dev, prod) or departments.
AZ-900 and AZ-104 require understanding the hierarchy: Tenant > Management groups > Subscriptions > Resource groups > Resources. Quota and billing limits apply per subscription.
Azure subscription is a concept that can show up on the exam. Check the full guide to see it in context.