This Project is a combination of different extensions, a patch, and integration into other extensions. For the extensions that are part of this project see Category:PerPageResources.
There are three different kinds of resources:
Files are the most obvious type of resource to attach to a page. The association to a page happens with a simple link to the page on the description-page of the file. So Media:Ubahn.jpg is associated with Main Page because the description links to it.
If the filename starts with a (sanitized version of the) page-name (i.e. Media:Main Page - Pruefung SS07.txt) that part is considered a prefix and the file is displayed and sorted without that prefix. In this example, the aforementioned file is listed and displayed under 'Pruefung SS07.txt' in Special:Resources/Main Page. This prefix is automatically prepended if you use the AddResource extension together with the mReferer-patch.
Subpages are normal MediaWiki-pages that reside under a 'primary' page. This is usefull if you want to split a project into several different parts or want to start a larger collaborative project that is still part of the primary project. Subpages are a normal part of MediaWiki, but you may have to configure $wgNamespacesWithSubpages if you use non-standard namespaces.
Special:Resources finds subpages with a modified version of the algorithm found in Special:Prefixindex. You can set the extension up so it also finds redirects, but it will never find broken redirects (this is important for the next chapter).
Sometimes you may want to add a link to a file or article that is hosted on an entirely different website. For that, we enhance normal redirects so they are also able to redirect to pages outside of your wiki. This functionality is provided with the ExternalRedirects-extension. The extension checks if a redirect starts with 'http://' and if yes, redirects to it. So external redirects are actually nothing else then broken 'normal' redirects.