meaning of "undefined" in script

Hi,

I am beginner to scripting. Generally, what does undefined mean in script. please explain in detail.

for example, in this script:

var myDocument = app.documents.add();
var myPageWidth = myDocument.documentPreferences.pageWidth;
var myPageHeight = myDocument.documentPreferences.pageHeight;
with(myDocument.pages.item(0)){
//Place guides at the margins of the page.
guides.add(undefined, {orientation:HorizontalOrVertical.vertical, <lb>
location:marginPreferences.left});
guides.add(undefined, {orientation:HorizontalOrVertical.vertical, <lb>
location:(myPageWidth - marginPreferences.right)});
guides.add(undefined, {orientation:HorizontalOrVertical.horizontal, <lb>
location:marginPreferences.top});
guides.add(undefined, {orientation:HorizontalOrVertical.horizontal, <lb>
location:(myPageHeight - marginPreferences.bottom)});
//Place a guide at the vertical center of the page.
guides.add(undefined, {orientation:HorizontalOrVertical.vertical, <lb>
location:(myPageWidth/2)});
//Place a guide at the horizontal center of the page.
guides.add(undefined, {orientation:HorizontalOrVertical.horizontal, <lb>
location:(myPageHeight/2)});
}