An "HTTP 404 Error" occurs when a web browser cannot find the requested page on a server. This error indicates that the server is reachable, but the specific resource, such as a webpage or file, does not exist at the given URL. Users often see a message stating that the page is not found. The 404 error can happen for various reasons, including broken links, deleted pages, or incorrect URLs. Website owners can customize the 404 error page to provide helpful information or redirect users to other parts of the site, improving the overall user experience.