Azure Subscriptions are (to me at least) an interesting concept/term. I've taken several Microsoft exams involving the topic but to date, I haven't necessarily found a succinct way to describe them. This description in SC-100 training documentation is one of the best I've seen:
Everything starts with a subscription, which can be considered a management unit for new application development and accelerating the migration of existing applications. It is important to align the subscription with business needs. Even if you have multiple subscriptions, they should be provided to support existing and new workloads.
(Source: Microsoft Cybersecurity Architect Exam Ref SC-100)