Que veux dire se script

<script language="JavaScript">
<!--
function na_preload_img()
{
var img_list = na_preload_img.arguments;
if (document.preloadlist == null)
document.preloadlist = new Array();
var top = document.preloadlist.length;
for (var i=0; i < img_list.length; i++) {
document.preloadlist[top+i] = new Image;
document.preloadlist[top+i].src = img_list[i+1];
}
}

function na_change_img_src(name, nsdoc, rpath, preload)
{
var img = eval((navigator.appName.indexOf('Netscape', 0) != -1) ? nsdoc+'.'+name : 'document.all.'+name);
if (name == '')
return;
if (img) {
img.altsrc = img.src;
img.src = rpath;
}
}

function na_restore_img_src(name, nsdoc)
{
var img = eval((navigator.appName.indexOf('Netscape', 0) != -1) ? nsdoc+'.'+name : 'document.all.'+name);
if (name == '')
return;
if (img && img.altsrc) {
img.src = img.altsrc;
img.altsrc = null;
}
}

// -->
</script>