He's talking about visual resources (e.g. graphics), and about CSS files. A "theme" folder would exist for each theme that can be selected. This theme folder would contain one or more CSS files and images for headers, sidebars, backgrounds, whatever. Each element in a theme folder has a name, and the corresponding element in any other theme folder must also have the same name, so the script can call, for example, "/$path_to_theme_folder/header.png" and actually find the file there ;-)