A 404 page is a standard response code in HTTP, displayed when a browser requests a webpage that the server cannot find. It's known as a 404 error, indicating that the server is reachable, but the specific page is not. Custom 404 pages are often used by websites to provide a more user-friendly experience. Instead of displaying a generic error, these custom pages can guide users back to working parts of the site, offer links to popular pages, or even incorporate creative design elements to lighten the mood. A well-designed 404 page can improve user experience, keep visitors on the site, and reduce bounce rates.