i can't figure out this code that's supposed to make an image that changes when the mouse is over it

<SCRIPT TYPE="text/javascript">

<!--

var rollOverArr=new Array();
function setrollover(OverImgSrc,pageImageName)
{
if (! document.images)return;
if (pageImageName == null)
    pageImageName = document.images[document.images.length-1
].name;
rollOverArr[pageImageName]=new Object;
rollOverArr[pageImageName].overImg = new Image;
rollOverArr[pageImageName].overImg.src=OverImgSrc;
}

function rollover(pageImageName)
{
if (! document.images
)return;
if (! rollOverArr[pageImageName])return;
if (! rollOverArr[pageImageName].outImg)
    {
    rollOverArr[pageImageName].outImg = new Image;
    rollOverArr[pageImageName].outImg.src = document.images
[pageImageName].src;
    }
document.images[pageImageName].src=rollOverArr[pageImageName].overImg.src;
}

function rollout(pageImageName)
{
if (! document.images)return;
if (! rollOverArr[pageImageName])return;

document.images[pageImageName].src=rollOverArr[pageImageName].outImg.src;
}
//-->
</SCRIPT>