Parallax scrolling is a web design trend where the background content – image or more and more often video, is moved at a different speed than the foreground content while scrolling.
There are many different degrees of parallax scrolling, from a complete site with things jumping out when you scroll down the page, to a relatively simple static background with foreground scrolling. This effect now pretty often are used in Premium WordPress themes, so you can check these themes too to get more inspiration.