Windows (and most other operating systems) have a "user land" and a "kernel space".

"user land" is where all your applications run. A "user land" application can only see other applications and files owned by the same user. Eventually, a user land app will want to do "something". This can be something like read a file from disk, make a network connection, draw a picture on the screen. To accomplish this, the user space app need to "talk" to the kernel.

If user space apps were instruments being played in an orchestra, the kernel would be the conductor. The kernel is responsible for making sure the user land apps can only see their respective users files/apps/etc.

The kernel "can see and do everything", it reports to no one. It has complete access to all the applications and every file. Your device drivers for your printer, video card, ect all run in "kernel space".

Basically, the OPs link: they've ported Doom to run effectively like a device driver. This means that if doom crashes, your PC will blue screen.

This has no practical purpose, other than saying "yeah, we did it" :)