[มือใหม่ PHP] ไม่เข้าใจคำสั่ง require_once ช่วยดูหน่อยคะ

if ($_GET['q'] && $_GET['maxResults']) {
  // Call set_include_path() as needed to point to your client library.
  'Google_Client.php';
  require_once 'contrib/Google_YoutubeService.php';

  /* Set $DEVELOPER_KEY to the "API key" value from the "Access" tab of the
  Google APIs Console <http://code.google.com/apis/console#access>
  Please ensure that you have enabled the YouTube Data API for your project. */
  $DEVELOPER_KEY = '1234';

  $client = new Google_Client();
  $client->setDeveloperKey($DEVELOPER_KEY);
  $youtube = new Google_YoutubeService($client);