This is a Java servlet filter (as per Servlet API 2.3). The idea behind this filter is very simple. Filter lets you stop downloading files from your site by users that are not visiting your site itself. For example, if someone tries to download a file from your site: http://your_site/your_file.zip by click on that link on another site: http://some_site/list_of_files.htm. In this case REFERER header in HTTP request points to some_site. And vice versa if your visitor clicks on the same link on your site REFERER header points to your site. So filter lets you set a mandatory value for REFERER header and redirect all the policy violated requests to some page.