nth-child是什么意思
我有教课书,看不懂,

代码不加nth-child 之前
 $('table  tr:even').addClass('even');  'even'类效果是蓝色背景
        <table >
            <tr>                 <td>odd</td>            </tr>// 此行变蓝
            <tr>                <td>even</td>            </tr>  
            <tr>                <td>odd</td>            </tr>// 此行变蓝
             <tr>                <td>even</td>            </tr>        
        </table>
以上我想把even行变蓝odd行却变蓝了,我已知道原因是因为 jquery下标从0开始.
但下面我改了代码成:
  $('table  tr:nth-child(even)').addClass('even');
然后就成了下面的效果
        <table >
            <tr>                 <td>odd</td>            </tr>// 
            <tr>                <td>even</td>            </tr>  //此行变蓝
            <tr>                <td>odd</td>            </tr>//        
                <tr>    <td>even</td>            </tr> //       此行变蓝
        </table>