这个练习真的搞不定啊，现成的代码也不大能看懂，求大神解释一下每一条啥意思
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>浏览器对象</title>
</head>
<body>
<h1>操作成功</h1>
<span id='second'>5</span><span>秒后回到主页</span>
<a href="javascript:back();">返回</a>
<script type="text/javascript">
var time=5;
var sec=document.getElementById('second');
function second () {
time--;
sec.innerHTML=time;
if (time==0) {
location.assign("http://www.imooc.com",'blank');
};
}
setInterval(second,1000)
</script>
</body>
</html>
