https://www.pcgamer.com/this-is-googles-custom-controller-for-its-stadia-streaming-game-service/

Google just announced a new game streaming service called Stadia, which promises to deliver 60 fps gaming at 4K. You will be able to use existing USB controllers with Stadia (or mouse and keyboard), though Google has designed its own custom controller with a couple of unique buttons.

The first of those is a capture button. Pressing it will making sharing content to YouTube quick and easy. You can also use this button to save content for yourself, or to share with friends (perhaps through Facebook and Twitter, though Google did not go into detail here).