I assume you're referring to an error message reported by your Web browser. If that's the case, it simply means that the Web browser was able to submit a request to the Web server, but didn't receive a response. This can (but shouldn't) happen if the Web server is overloaded. I say it shouldn't happen, because what should happen is the Web browser should return a 500 error indicating that it's too busy. Other possible causes of this error are communication problems (for example, dropped packets) or a Web page that performs time-consuming processing (for example, a page that performs database maintenance that takes ten minutes).
This was first published in June 2004