CSS on click circular transition

How could I create a circular transition like the one that happens in this page (for instance, when you click the green square)? I know it involves transitions and animations, but I´m a bit lost:

what would a “circular transition” be?
You could make the whole background a circle that becomes smaller once you click something, and everything else fades out to reveal new (possibly secret) content?

Do you just mean going from a square to a circle?