A gateway or proxy server timed out without responding. The browser has requested a specific transfer protocol version that is not supported by the server. The server should return what protocols are supported. Users get frustrated by error messages that don't really tell them anything. Even the descriptions above for the various return codes don't say what you, the user, can do.
Webmasters can help.
By analyzing their logs a webmaster can determine which error codes are being returned to users. For the most common, more descriptive error messages can be generated and the system told to use them. Use whichever is appropriate to help users when they encounter errors at your site. If nothing else, include a 404 ErrorDocument command to help those who mistype something. These cause requests to specific files that have been moved to be automatically directed to their new location and gives feedback to the search engines that the URL has changed.