The part after index.php after the question mark is referred to as a query string. This is a way to supply additional request information to a PHP file. It's typically used when you want to tell the PHP script additional details about the information you are requesting. This information will show up inside your PHP script in the [man]$GET[/man] superglobal array. You could try putting this in index.php to see what information might have been supplied in the query string:
var_dump($_GET);
Or you can simply output the query string like so:
echo $_SERVER["QUERY_STRING"];