This is the second specification for Web Components and this one is responsible for encapsulation. Both the CSS and DOM can be encapsulated so that they are hidden from the rest of the page. What a shadow DOM does is let you create a new root node, called shadow root, that is hidden from the normal DOM of the page.
However, even before we jump into the concept of a shadow DOM, let's try to look at what a normal DOM looks like. Any page with a DOM follows a tree structure. Here I have the DOM structure of a very simple page:
In the preceding image, you can see that #document is the root node for this page.