The dock is plugged in to the AC outlet.
This “thing” gets plugged to the dock via a USB-C cable (included in the box).
So you can charge while playing and have the switch charge as well.
To be honest though, I think the included cable is a bit short to play on tv while it’s recharging. It looks like it’s about 3ft long if not shorter. I would plan on using a usb extenders to do that!