'defaults' => array('field'=>'title') in hook_views_data ?

This part seems hard to understand

 'defaults' => array(
      'field' => 'title',
    ), 

inside this:

function node_views_data() {
  
  // node table -- basic table information.
  // Define the base group of this table. Fields that don't
  // have a group defined will go into this field by default.
 
$data['node']['table']['group'] = t('Content');
// Advertise this table as a possible base table
  $data['node']['table']['base'] = array(
    'field' => 'nid',
    'title' => t('Content'),
    'weight' => -10,
    'access query tag' => 'node_access',
    'defaults' => array(
      'field' => 'title',
    ),
  ); 

from this website：http://www.grasmash.com/article/demystifying-views-api-developers-guide-...　

Actually this part also difficult to understand

'access query tag' => 'node_access',

Can some one help us make it clear ?

Thanks in advance.

Regards. and Thanks.