What is cookiehash?

What the hell is the $cookiehash variable and why is it so important?
Example of where its used:

if((!empty($_COOKIE['wordpressuser_'.$cookiehash])) && (!empty($_COOKIE['wordpresspass_'.$cookiehash]))) {
$user_login = $_COOKIE['wordpressuser_'.$cookiehash];
$user_pass_md5 = $_COOKIE['wordpresspass_'.$cookiehash];
}