这几行代码是什么意思？

 window.onload = function() {
        var oUl1 = document.getElementById("ul1");
        var aLi = oUl1.getElementsByTagName("li");
        var oDiv = document.getElementById("tab-list");
        var aDiv = oDiv.getElementsByTagName("div");
        for(var i = 0; i < aLi.length; i++) {
            aLi[i].index= i;
            aLi[i].onmouseover = function() {
                for(var i = 0; i < aLi.length; i++) {
                    aLi[i].className = "";
                }
                this.className = "active";
                for(var j = 0; j < aDiv.length; j++) {
                    aDiv[j].className = "hide";
                }
                aDiv[this.index].className = "show";
            }       
        }
    }
   1、aLi[i].index = i 2、aLi[i].className = "" 3、 this.className = "active"; 4、 aDiv[this.index].className = "show"; ，特别是第三条的this指的是什么