Definition - What does HTTP 404 mean?
HTTP 404 is a Hypertext Transfer Protocol (HTTP) standard error message response code that simply means "not found," indicating that a Web server is unable to locate the resource requested by a client, usually a Web browser. This is often because of a dead or broken link that has not updated removed or transferred contents to point to the new location. The server then generates a 404 Not Found Web page, which is sent as a notification to the client for the user.
Techopedia explains HTTP 404
When the error code is sent by the server as a response, it is accompanied by a human readable "reason phrase," indicating the error type, which in this case is "Not Found." Another error is 410, which means "Gone," while 301 means "Moved Permanently." Both require server configuration, so by default, the majority of websites simply use 404 in their place.