This is a Java servlet filter (as per Servlet API 2.3). This filter lets you proceed If-Modified-Since header in the request. The If-Modified-Since header field is used with the GET method to make it conditional: if the requested resource has not been modified since the time specified in this field, a copy of the resource will not be returned from the server. In this case a status code 304 (not modified) will be returned with empty body. So you can save bandwidth for static files for example.
This filter aslo sets Last-Modified header in the response.