“百度关键词跟踪代码”是什么？

如下：

<!-- 百度关键字跟踪代码1 start-->
<script type="text/javascript">
var _bdhm_top = 0;
var _bdhmProtocol = (("https:" == document.location.protocol) ? " https://" : " http://");
var _bdhm_tim = new Image(1,1);
_bdhm_tim.id = "bdhmPerimg";
_bdhm_tim.src = _bdhmProtocol + "hm.baidu.com/_tt.gif?si=07a81d45ff030b63fe2a0f375b779f09&rnd=" + Math.round(Math.random()*2147483647);
_bdhm_tim.onload=function(){_bdhm_top = 1;}
</script>
<!-- 百度关键字跟踪代码1 end-->