A 404 error, also known as a "page not found" error, is an HTTP status code that indicates that the server cannot find the requested resource or page. This can happen when a user clicks on a broken link or types in an incorrect URL. 404 errors can be frustrating for users, but they can also be harmful to SEO if they are not properly managed. Website owners can customize their 404 pages to provide helpful information to users and to prevent them from leaving the site. It is also important to regularly monitor 404 errors and fix broken links to ensure that users can access the content they are looking for. In addition, website owners can use tools such as Google Search Console to identify 404 errors and to redirect users to the correct page or resource.