What is the css html root element

Question

I recently started using NetBeans IDE (6.9.1) and I've already added a stylesheet to my under construction website with it.

I was surprised to see that one thing was automatically added:

root { 
    display: block;
}

I could discover some information on the:root pseudo-class but nothing specific about the root element itself when I looked around.

What is the root element and what purpose does it serve?