谁能解释下布局下面五个index啥意思

<div id="container">

<div id="list" style="left: -600px;">

<img src="img/5.jpg" alt="1" />

<img src="img/1.jpg" alt="1" />

<img src="img/2.jpg" alt="2" />

<img src="img/3.jpg" alt="3" />

<img src="img/4.jpg" alt="4" />

<img src="img/5.jpg" alt="5" />

<img src="img/1.jpg" alt="5" />

</div>

<div id="buttons">

<span index="1" class="on"></span>

<span index="2" ></span>

<span index="3" ></span>

<span index="4" ></span>

<span index="5" ></span>

</div>

<a href="javascript:;" id="prev" class="arrow">&lt;</a>

<a href="javascript:;" id="next" class="arrow">&gt;</a>

</div>