This isn't going to seem much like anything special if you can't see this visually, sorry! In the background of this page are swoopy arcs with purple gradients, starting with a fairly high contrast within the gradients, and becoming more uniformly purple as you scroll down.
However as you scroll, the part with this text in it remains fixed in place, with the background flowing up underneath it. The background of this text-filled div blurs, brightens and pulls down the contrast of the swoopy puple arcs, appearing a little like frosted glass. You may consider it a commentary on Apple's Big New Thing in user interfaces. The text could certainly be more readable...