The MSIE browser supports a special kind of window called a Modal Dialog. This is similar to using the open() method to create a new window except that it is forced to always be on the top and you cannot click outside of it to bring focus to another window within the browser.
When the modal window exits, you have the opportunity to store a value in the returnValue property. Since the window is instantiated by a method call, the caller will expect a value to be returned. This value will be null unless you specify something yourself.
Since you can pass arguments into the modal window, this provides a way to transmit user interaction dependent values back to the caller.
How this behaves in a truly multi-process environment such as Windows NT or Mac OS X may be slightly different. In an operating system such as those, modality may only extend within the application and it may be possible to switch to a different application. However on returning to the MSIE browser, the modality will still be in force.