Eh, it's not much of an explanation. If you've done any WinForms development, you know that for each form you have two files: one that has "Designer" in its name that specifies what controls are on that form and where they're located along with any other properties you set through the designer. Then there's the so-called code-behind, which is where you handle events raised by the controls.
WPF instead separates the layout of a form, which you design with a XAML file, from the implementation (the behavior, as that tutorial calls it). So theoretically you could have one guy design the form, and a different guy write its implementation, which would be doable but impractical in WinForms. As an approximation, you can think of how HTML defines what's in the page, and CSS defines what it looks like.
I suggest you don't get hung up on that ".NET instances", it doesn't mean a whole lot. There must be tutorials on YT that are better put together than those on that website.
Already the first paragraph of that section says:
One of the longstanding problems that all of us face with GUI design can be solved by using XAML. It can be used to design UI elements in Windows Forms applications.
If by Windows Forms they mean what we call WinForms, then that's patently false, because XAML is for WPF. If they mean a generic "windows application with forms", then that's correct.
If you lack historical context, WinForms came out first, and WPF after a few years.