Interestingly, this new feature follows news of Microsoft reportedly skipping HoloLens V2 to create the third iteration instead, and murmurs that Xbox One and Scorpio will be getting mixed-reality headset support in 2018.
In order to access the feature, you need to of course be running the 15048 build. You then need to open up the Update & Security section of Windows' Settings screen, and click on "For Developers" on the left side. From there, you can choose the developer mode option, and subsequently enable it.
Because you've enabled developer mode, you've now revealed the Mixed Reality app in the Start menu. After opening, you'll see a screen like the one above, and after clicking about for a bit, you'll be able to see if your PC is capable of dislaying mixed-reality content, and if you've been blessed with a Mixed Reality dev kit, you could proceed to test with the demo. Otherwise, you'll have to make due with the simulation. For control, WASD will move you around, while your mouse will help you look in the right direction.
Given how hidden this feature was in the latest build, it goes without saying that this is just a teaser for now. By the time this portal rolls out to stable builds, we're likely to see its capabilities greatly improved.