Press dot to record game in Hearthstone?

Sometimes when I start Hearthstone, it has a brief popup message that says "Press <> to record your game" where <> is just a large black dot.

What does this mean? I see no black dot in the Hearthstone interface.

I think that is your Windows 10 installation's DVR, which is part of its XBox app.

The message popping up when starting a game looks like a typical Windows 'ribbon', coming in from either the left or right side of the screen, and has a • symbol to denote your (customizable) 'record' option (Win+g by default).
The symbol refers to your 'game bar', where you can start and stop recording: