This is a Java servlet filter (as per Servlet API 2.3). This filter lets you allow cross domain Ajax requests to your resources. Firefox 3.5 allows you to make XMLHttpRequests cross domain (in other words code loaded from some domain foo.com can make XHR requests to bar.com). Mozilla's spec offers a special HTTP header for that. This filter lets you add such a header to your site so cross domain requests could be allowed for existing web applications.