请问伙伴们：一段 PHP 代码的含义

<div class="aw-mod aw-topic-bar" id="question_topic_editor" data-type="question" data-id="<?php echo $this->question_info['question_id']; ?>">
<div class="tag-bar clearfix">
<?php foreach($this->question_topics as $key => $val) { ?>
<span class="topic-tag" data-id="<?php echo $val['topic_id']; ?>">
<a href="topic/<?php echo $val['url_token']; ?>" class="text"><?php echo $val['topic_title']; ?></a>
</span>
<?php } ?>

<?php if ($this->user_id AND ((!$this->question_info['lock'] AND $this->user_info['permission']['edit_topic']) OR $this->user_id == $this->question_info['published_uid'])) { ?><span class="icon-inverse aw-edit-topic"<?php if (sizeof($this->question_topics) == 0) { ?> style="display:none"<?php } ?>><i class="icon icon-edit"></i></span><?php } ?>
</div>
</div>


这段代码中<?php foreach($this->question_topics as $key => $val) { ?> 这段代码如何理解？遍历的是什么地方？ question_topics 是一个字段吗？_topics 前面的下划线什么意思？ question 又代表什么意思？