From the FAQ on the AirParrot site:
What does AirParrot do?
AirParrot lets you AirPlay your Mac’s screen to a second or third generation AppleTV. What you see on your Mac’s screen will appear on the AppleTV, wirelessly!
How do I use AirParrot?
Once you’ve opened AirParrot, click on the icon in your menu bar. Select the AirPlay device (such as your AppleTV) and then select which screen you want to mirror. To stop mirroring, simply click on the same AirPlay device in the menu!