304 is the HTTP status response for "not modified." It is used in our log files to indicate that the logged request was actually served by some other part of our network. Usually only simple files, like static graphics and files, are eligible for this type of handling. Dynamic content, such as PHP and CGI scripts, will typically be served (and logged) in its entirety unless your application generates appropriate cache-eligible headers and 304 responses.
This code can also appear for other reasons, such as if someone accesses your site through a web cache.