Containers that organize multiple Azure subscriptions into a hierarchy so policies and access control can be applied consistently. The topmost level of Azure's management hierarchy.
The top of the hierarchy in AZ-104 and AZ-900: Management Groups > Subscriptions > Resource Groups > Resources. Policies and RBAC applied at a management group are inherited downward.
Management groups (Azure) is a concept that can show up on the exam. Check the full guide to see it in context.