This is a pretty awesome feature, which will allow you to cast anything that is on your Android device's screen. Google demoed it on stage by showing off Google Earth as well as a live stream from a smartphone camera to a TV. From the Chromecast app, you can start screen mirroring from the navigation tray, and on Nexus devices, the option will be in the Quick Settings menu. The only downside right now is in the compatibility. As was said during the keynote, only a select group of devices are compatible at first, including the Nexus lineup, Samsung Galaxy S4, S5, and Note 3, LG G Pro 2, G2, and G3, and the HTC One (M7).
More devices are said to be added soon, but it is still unclear what is required to make this work, so it's hard to say what devices will be supported. We will keep an eye out for more updates on this. The new version with the screen mirroring is 1.7.
source: Official Android Blog