How do MySQL queries in PunBB work?

<?php
define('FORUM_ROOT', 'forum/');
require FORUM_ROOT.'include/common.php';

define('RECENT_POSTS_SHOW_POST', true); // Set to false to show topic subject only

$query = array(
    'SELECT'    => 'p.id, p.message, t.subject, t.poster, t.num_replies, t.posted, p.poster_id, t.num_replies',
    'FROM'        => 'topics AS t',
    'WHERE'        => 't.forum_id = 2 AND t.sticky = 0',
    'JOINS'        => array(
        array(
            'LEFT JOIN'        => 'posts AS p',
            'ON'            => 'p.topic_id = t.id'
        )
    ),
    'ORDER BY'    => 't.posted DESC',
    'LIMIT'        => '0,10'
);
 
$result = $forum_db->query_build($query) or error(__FILE__, __LINE__);
 
$recent_posts = array();
while ($cur_post = $forum_db->fetch_assoc($result))
    $recent_posts[] = $cur_post;
 
// Print out posts
if (!empty($recent_posts))
{
    foreach($recent_posts as $cur_post)
    {
        $message = str_replace("\n", "<br />", $cur_post['message']);
        $date = date("j  M  Y", $cur_post['posted']);
        
        echo '
        <h2>'.$cur_post['subject'].'</h2>
        <p class="info">Written by <a href="'. FORUM_ROOT .'profile.php?id='.$cur_post['poster_id'].'" 
        title="'.$cur_post['poster'].'\'s profile">'.$cur_post['poster'].'</a> on '.$date.' | 
        <a href="'. FORUM_ROOT .'viewtopic.php?id='.$cur_post['id'].'" title="View Replies" >'.$cur_post['num_replies'].' Comments</a></p>
        <p>'.$message.'</p>';
        
    }
}
 else { echo '<p>We are sorry but no news were found in the database, if you believe this is an error, please notify the administrator.</p>'; }
?>