prefetch requests - what is it?

// Block prefetch requests
if (isset($_SERVER['HTTP_X_MOZ']) && $_SERVER['HTTP_X_MOZ'] == 'prefetch')
{
    header('HTTP/1.1 403 Prefetching Forbidden');

    // Send no-cache headers
    header('Expires: Thu, 21 Jul 1977 07:30:00 GMT');    // When yours truly first set eyes on this world! :)
    header('Last-Modified: '.gmdate('D, d M Y H:i:s').' GMT');
    header('Cache-Control: post-check=0, pre-check=0', false);
    header('Pragma: no-cache');        // For HTTP/1.0 compability

    exit;
}