What does it mean to have a variable between parentheses after a function declaration?

Could you please explain what $arg means in this piece of code? (it's from a Drupal module)

function node_add_review_load($arg) {
  global $user;
  $add_review = FALSE;
  $current_node = node_load($arg);
  $type =$current_node->type;
  $axes_count = db_result(db_query("SELECT COUNT(*) FROM {nodereview_axes} WHERE node_type='%s'", $type));
    if (variable_get('nodereview_use_' . $type, 0) && $axes_count) {
      $add_review = db_result(db_query("SELECT n.nid FROM {node} n INNER JOIN {nodereview} nr ON n.nid=nr.nid WHERE uid=%d AND reviewed_nid=%d", $user->uid, $arg));
    }
    return $add_review ? FALSE : $arg;
 }

In general, arg is short for "argument," as in "an argument to a function." It's a generic, and thus unhelpful, name. If you'd just given the method signature (function node_add_review_load($arg)) we'd have no idea.

Fortunately, with the complete function body, we can deduce its purpose: it is the node_id. This function loads the node identified by $arg and then tries to find a corresponding row that's loaded, and that the code then tries to find a corresponding review for the current user. If successful, the function will return that same node_id (i.e., $arg); otherwise it will return FALSE.