Not Modified

The cached version is still valid, so no need to re-send the resource.

304 Not Modified is part of HTTP caching. When a client sends a conditional request with If-None-Match or If-Modified-Since, the server replies 304 if the cached copy is still fresh, with no body. That saves bandwidth and speeds up the page. Seeing lots of 304s in your logs is usually a good sign that caching is working.