What is comments-popup.php Used For?

Hi,

I am basically creating my own Theme based off the default Kubrick theme provided. It’s pretty much completed, but I can’t see where comments-popup.php is actually used?

What is is used for and how can I see an example of it in action??

I did read a bit in the Codex but I couldn’t really get my head around it.

Thanks to any help in advance.