I'm guessing you've read this topic already, right?

Other than that, there's not a whole lot more to understand.  The html file sets the main look of the forums, the BoardIndex file sets the look of your forum's front page, MessageIndex sets the look of the page you see when you click on a specific board, Display sets the look of the page you see when you view a topic and HelpCentre sets the look of your help pages.

The pages are mostly written in HTML.  Some of the elements are kind of mixed up (they define the list of buttons at the top of the page, and then use a <yabb> tag to place those buttons in the right place within the template, etc.)