you must have one of the macs in the article that GGJstudios linked to. basically within about 2 years old.
If you are planning on watching video using this, the quality will be lower and the video will probably stutter a bit.
Wired ethernet or fast wi-fi (802.11n 5GHz) will eliminate some but not all of this.
when it comes out in a few months OS X Mavericks will include the option to use airplay as a second display instead of just showing the same image as on the main display. It will probably have the same quality issues.