Azure Managed Disks are the default disks selected when you create a VM in the Azure Portal. They handle storage for your virtual machines completely. Previously, you would have to manually create storage accounts to store VM hard disks, and when your VM needed to scale up, you have to add additional storage accounts to make sure you didn't exceed the limit of 20,000 IOPS per account.
With Managed Disks, this burden is now handled for you by Azure. You can now create 10,000 VM disks inside a subscription, which can result in thousands of VMs inside a subscription, without the need to copy disks between storage accounts.
If you are still using Unmanaged Disks, it is highly recommended that you switch to Managed Disks. To convert your disks from Unmanaged to Managed, refer to the following article: https://docs.microsoft.com/en-us/azure/virtual-machines/windows/convert-unmanaged-to-managed-disks.