With each release of Windows 10 Insider Preview builds, more and more advanced features are being introduced. As Microsoft preps for the first public preview of Hyper-V Containers, Windows 10 build 10565 introduced a preview of nested virtualization. Nested virtualization is a feature that virtualizes certain hardware features that are required to run a hypervisor in a virtual machine.
While Hyper-V Containers are not officially out yet, nested virtualization allows for you to try it out on Hyper-V virtualization machines. As I am not the most equipped to explain nested virtualization, Microsoft's Virtualization Blog has a better explanation:
"Hyper-V relies on hardware virtualization support (e.g. Intel VT-x and AMD-V) to run virtual machines. Typically, once Hyper-V is installed, the hypervisor hides this capability from guest virtual machines, preventing guests virtual machines from installing Hyper-V (and many other hypervisors, for that matter). Nested virtualization exposes hardware virtualization support to guest virtual machines. This allows you to install Hyper-V in a guest virtual machine, and create more virtual machines “within” that underlying virtual machine."
For more information about the known issues with nested virtualization or the steps to enable nested virtualization, head over to Microsoft's Virtualization Blog.