Here
this.returnValue
is the value returned from that target file showing on Modal.
On that file if you specify what to return, then this will contain the return value like below.
<script type="text/javascript">
function closeMe() {
window.returnValue = 'The Return Value + ' + window.dialogArguments;
window.close();
}
</script>
Here it is returning the message as "The Return Value" appended with the arguments you sent.
You have sent
window
as the arguments.
Refer -
IE window.showModalDialog() - Cross Browser solution[
^].
Go through the article and you will understand completely.