Although a site may not always warrant a full multilayer navigation menu, most websites usually have at least a few different sections. Often many pages have links back to the top pages of each section, and therefore it is useful to highlight which section the user is currently in. Listing 9.2.1 looks at the URL of the current web page and, based on that, determines what section of the website it is in and changes the section menu.
They provide the script and a brief explaination, but not much else. Seems like the snippet might make more sense with a little more context, but it's still useful.