When you purchase an Azure Reservation you choose a scope for it which defines where it may be used: within a specific Resource Group, a specific Subscription, or all Subscriptions. A Management Group is a group of Azure Subscriptions and Microsoft announce this as a new option for the scope of a Reservation.
Find the announcement on the public preview here: https://bit.ly/3iqyA5i and more information on Reservation scopes generally here: https://bit.ly/3ouf6AU.