This Technical White Paper covers how to build a nested lab with NSX-T 2.3 running in it, so that you can easily explore the VMware NSX platform. A nested hypervisor is the process of running a hypervisor inside another hypervisor… or an ESXi host in a virtual machine, running on an ESXi host.
A nested lab is like mini-datacenter that is a collection of virtual machines which provides a small area that won’t impact your production environment and doesn’t require a lot of expensive hardware or space, hence is extremely useful and cost-effective.
Also take a look at Why Everyone Needs an NSX Nested Lab/Sandbox