304 Not Modified is the HTTP status code returned to the client from the server-side. It tells a client application, such as a browser, or an application on that client, that the file requested by the client has NOT been changed since it was last accessed. Usually, this comparison is based on the Last-Modified header sent from the server and the copy on the client to see if there was any change in the file's last-modified time. 304 status code is returned when there is NO change in the requested file's Last-Modified header.