Subscription can be considered as billing unit that aggregates all costs of underlying resources.
It Contain resource group and their associated resources.
Scoping level for governance and security
You can create multiple subscription
User can have access to one or more subscriptions with different roles
Subscription can be used to organize recourses into completely distinct accounts.
Resource consumed by subscription will be charged to owner. If subscription will not be paid all resource within subscription will be stopped.
Subscription can be only associated with a single org (Azure AD tenant) at a time. {Switch AD Tenant Option}