简单描述： 用到的知识点 css 中的绝对定位 以及 Js 中的事件冒泡（或事件委托）
.cont{display:inline-block;width:200px;height:200px;border:1px solid red;position:absolute;left:150px;top:2px;display:none;}
<div style="border:1px solid green;margin:30px auto;width:150px;" id="menu">
<div class="item" id="div1" style="position:relative;">
<span style="display:inline-block;background: #abcdef;width:150px;height:50px;cursor:pointer;">123</span>
<span id="span1" class="cont">
hello1111111
hello1111111
hello1111111
hello1111111
hello1111111
</span>
</div>
<div id="div2" class="item" style="position:relative;">
<span style="display:inline-block;background: red;width:150px;height:50px;cursor:pointer;">123</span>
<span class="cont">hello222222</span>
</div>
<div id="div3" class="item" style="position:relative;">
<span style="display:inline-block;background: green;width:150px;height:50px;cursor:pointer;">123</span>
<span class="cont">hello333333</span>
</div>
</div>
window.onload = function(){
var oSpan1 = document.getElementById("span1");
var oMenu = document.getElementById("menu");
var oSMenu = oMenu.childNodes;
for(var n=oSMenu.length-1; n>=0; n--){
if(oSMenu[n].className == "item"){
oSMenu[n].onmouseover = function(evt){
var target = oTargetByClass._get_target_child(this.childNodes, "cont");
target.style.display = "block";
}
oSMenu[n].onmouseout = function(evt){
var target = oTargetByClass._get_target_child(this.childNodes, "cont");
target.style.display = "none";
}
}
}
var oTargetByClass = {
_get_target_child: function(childNodes, child_class){
for(var i = childNodes.length-1; i>=0; i--){
if(childNodes[i].className == child_class){
return childNodes[i];
}
}
}
}
}