Microsoft Azure Reservations are a way of pre-paying for an Azure resource over a one or three-year term to get the most cost-effective pricing. The Reservations family continues to grow: it started with Reserved Instances, a way of pre-paying for Azure virtual machine base compute, and was extended with Reserved Capacity, a way of pre-paying for Azure SQL Database compute capacity.
Now the family also includes Software Reservations for SUSE Linux software and Reserved Capacity for Azure Cosmos DB throughput.
The following link gives you an overview of Reservations and if you expand the “Buy a reservation” link at the left you can get details on the current four members of the Reservations family: https://docs.microsoft.com/en-us/azure/billing/billing-save-compute-costs-reservations.