我是新手能帮我解释一下这句的意思吗
<td   id= "imgmenu1 "   class= "menu_title "   onmouseover= "this.className= 'menu_title2 '; "   onclick= "showsubmenu(1) "   onmouseout= "this.className= 'menu_title '; "   style= "cursor:hand "   background=image/menudown.gif   height= "25 ">  
            <span> 系统管理   </span> </td>

------解决方案--------------------
onmouseover= "this.className= 'menu_title2 '; " 鼠标移入后调用menu_title2
onclick= "showsubmenu(1) " 鼠标点击后调用一个js函数
onmouseout= "this.className= 'menu_title ' 鼠标移开后调用menu_title