What does the return code 404 mean in an HTTP header?

The return code 404 in an HTTP header signifies that the requested resource could not be found on the server. It indicates that the server was reachable, but the specific page or file the client is trying to access does not exist at the given URL. This status code often arises when a user tries to navigate to a link that has been removed or a URL that was typed incorrectly. In web development, it's common to implement a custom 404 error page to guide users when they encounter this status, offering them options to return to the homepage or search for alternatives.