帮忙解释一下这段javascript的含义

function showHide(parent) {
var aDD=parent.getElementsByTagName('dd');

for(var dd=0; dd<aDD.length; dd++) {
aDD[dd].style.display=(aDD[dd].style.display=='none')? 'block' : 'none';

}
}
function addLoadEvent(func) {
var oldonload = window.onload;
if (typeof window.onload != 'function') {
window.onload = func;
} else {
window.onload = function() {
if (oldonload) {
oldonload();
}
func();
}
}
}

addLoadEvent(function() {
var aDT=document.getElementsByTagName('dt');
for(var i=0; i<aDT.length; i++) {
showHide(aDT[i].parentNode);
aDT[i].onmouseover=aDT[i].onmouseout=Function ('showHide(this.parentNode)');

}
});
showHide(aDT[i].parentNode);
aDT[i].onmouseover=aDT[i].onmouseout=Function ('showHide(this.parentNode)');