The Chromecast is a small HDMI stick that plugs in your television. It is programmable and has an SDK that allows you to write apps on your mobile phone to control the display from the app. The Chromecast Countdown is a generic timer for showing countdowns on televisions. It is useful for presentations, workouts, etc.
The Chromecast Countdown is quite a popular app on the play store, probably because of its simplicity. I myself use it regularly on the Show & Tell nights at the Hackers & Founders office.