As the name suggests, a subscription refers to the logical entity that provides entitlement to deploy and consume Azure resources. Imagine Azure subscriptions to be like a pre-paid SIM activation with or without credit. Depending on the type, these can be free subscriptions, Pay-As-You-Go (Post-Paid) subscription or a pre-paid credit carrying subscription.
A subscription fuels the Azure resources in a customer tenant. Everything will suspend or halt if the subscription carries zero credit unless the subscription is a post-paid pay-as-you-go subscription