On-demand capacity reservations for Azure Virtual Machines let you deploy and manage the compute capacity required to run Azure Virtual Machines separately from the virtual machines (VMs) themselves. This new feature enables your IT organization to reserve compute capacity for a VM size. The reservation can be for any length of time in any public Azure region or availability zone and supports most VM series. You can create and cancel an on-demand capacity reservation at any time, no commitment is required.
The ability for you to access compute capacity–with SLA guarantees–ahead of actual VM deployments is particularly important to ensure the availability of business-critical applications running on Azure. On-demand capacity reservations can be combined with Azure Reserved VM Instances (RIs) to significantly reduce costs.
Read the blog for more details. You can also watch this video or read the documentation.