Can someone explain what a Validation Environment is?

Hi jsebast1245, the validation environment is the canary ring and new Windows Virtual Desktop features like e.g. MSIX App Attach is enabled here first. 

With canary deployments, we deploy a new function set or code in a small part of the production infrastructure. Once the function is signed off for release, only a few users are routed to it. This minimizes any impact.

For your personal internal tests you can use a separate host pool. But new service feature will be available in the validation environment first.