A 404 error in web browsing signifies that the server cannot find the requested resource. This error indicates that the URL entered may be incorrect, the page may have been removed, or it no longer exists. Users encountering a 404 error are often redirected to a custom error page, guiding them to other useful sections of the website. For website owners, persistent 404 errors can indicate broken links, making it crucial to regularly monitor and maintain site content to enhance the user experience.