Hi. I have just bought a 64GB iPad and I am wondering what's the significance of an icon that looks like an open book or a page that is visible when you watch YouTube videos in full screen. It's to the left of the "play" button. It used to look clear, and out of curiosity I clicked it. I didn't notice any changes whatsoever except that the icon/symbol is now greyed out and nothing happens when I click on it again. Just what exactly does this icon represent?