代码里面的b1是什么意思？初学，见谅...

<html>
<head>
<script type="text/javascript">
function mouseOver()
{
document.b1.src ="/i/eg_mouse.jpg"
}
function mouseOut()
{
document.b1.src ="/i/eg_mouse2.jpg"
}
</script>
</head><body>
<a href="/index.html" target="_blank">
<img border="0" alt="Visit W3School!" src="/i/eg_mouse2.jpg" name="b1"  onmouseover="mouseOver()" onmouseout="mouseOut()" /></a>
</body>
</html>