That's the "Cast to screen" button in the mobile Youtube app. If you happen to press it the Youtube app will connect to the nearest available Smart TV, for example the one in the living room where your family is watching TV, and then suddenly they see whatever weid crap you're watching on Youtube.

My entire family keeps ruining my online matches with this. And my neighbour did it once, one moment I'm playing overwatch the next there's a bloody peppa pig video.