An error page informs a visitor when there is a problem accessing your site. Each type of problem has its own code. For example, a visitor who enters a nonexistent URL will see a 404 error, while an unauthorized user trying to access a restricted area of your site will see a 401 error.
Basic error pages are automatically provided by the web server (Apache). However, if you prefer, you can create a custom error page for any valid HTTP status code beginning in 4 or 5.