What's this statistics-looking icon on my TouchBar? I've just discovered this icon on my MacBook Pro's TouchBar:



Looks like some sort of statistics, or so? It only appears occasionally. Tapping on it I get this slider:



This doesn't do anything though. When sliding I get a "0:00" count next to it and nothing happens. What is this supposed to do?

Looks like I'm also unable to take TouchBar screenshots via ⌘+⇧+6 for some reason.
Are you watching a video in Safari? That slider is telling you that you're watching a video in the app displayed in the icon, scrubbing on that slider scrub the video you're playing. Or if you're not watching a video, it might just be a bug. Try refreshing the Control Strip by going into activity monitor and find the process "Control Strip" and quit that process(it won't do any harm, your touch bar will blink once). 

P.S. As for taking screenshot of the touch bar you need 10.12.2 beta.