A "404" error is an HTTP status code that indicates a webpage could not be found on a server. This typically occurs when a user tries to access a URL that no longer exists, has been moved, or was typed incorrectly. The server responds with a 404 error to inform the user that the requested resource is unavailable. Websites often display a custom 404 error page to guide users back to functional areas of the site. These pages may include links to the homepage or a search bar, helping users find the information they need despite the missing page.