最近刚学javascript，一直不是很理解二维数组，有大神能给我讲一下这个二维数组的意思吗？

Question

打印出都是女生的姓名

var infos=[

<script type="text/javascript">

var infos=[["小A","女",21,"大一"],["小B","男",23,"大三"],["小C","男",24,"大四"],["小D","女",21,"大一"],["小E","女",22,"大四"],["小F","男",21,"大一"],["小G","女",22,"大二"],["小H","女",20,"大三"],["小I","女",20,"大一"],["小J","男",20,"大三"]];

 //第一步把之前的数据写成一个数组的形式,定义变量为 infos

 for(var i=0;i<infos.length;i++){

 if(infos[i][3]=='大一'&&infos[i][1]=='女'){

 document.write(infos[i][0]+"<br/>");

   }

 }

这个什么i,infos[i][3]怎么去理解？