So what is it? Well, as per Codyhouse:
Alternate Fixed & Scroll Backgrounds
If you’ve been surfing a lot in the recent months (or years), there’s this some kind of “reveal” effect when you scroll thru a page. Here’s the demo to understand it more.
However, do note that this parallax technique might not work well when viewed using your phone or tablet. If you’d like to have a more advanced and bullet-proof parallax effect, you might want to see Skrollr.js.