The "HTTP 404 Not Found" is a standard response code in the Hypertext Transfer Protocol (HTTP) that indicates the server could not find the requested resource. This typically occurs when a user tries to access a webpage that no longer exists or has been moved without a proper redirect. When a browser encounters a 404 error, it displays a message to inform the user that the page is unavailable. This can happen for various reasons, such as a mistyped URL, broken links, or deleted content. Understanding this error helps users navigate the web more effectively.