According to TechCrunch:
Though we can't confirm the exact functionality of the app, it's likely to be focused on watching streams. Periscope made videos viewable on the web in June without the ability to broadcast, and the Apple TV app could work similarly.
The report offers no other details on the rumored app, and there's no word on when it might be launched. The article speculates that it might be demoed during Apple's press event in San Francisco on September 9.