Whenever your users enter an incorrect login name or password, an error message is displayed on the login page alerting them to an incorrect piece of information. If a hacker were to correctly guess one of those pieces of information, the error message would help them identify what he/she got right.

 

function ns_no_wordpress_errors(){
  return 'Something is wrong!';
}
add_filter( 'login_errors', 'ns_no_wordpress_errors' );