Menu Effect

1

Ok, I am a major newbie and I am sure that this is a very basic question, but I would really like some insight. I’ve seen this effect on many different sites on menus and I would really like to know how to do it or just know what it is called so I can look it up myself. Check out the way that the menus appear on this site: http://www.soloflow.com/ How are those masked (I guess) where the background is not covered up as well during the animation? I look forward to hearing your thoughts. Thanks!