You need a satellite or cable subscription to watch some content on the Apple TV. For example, you can watch some ABC content without a subscription, but if you want to watch the ABC network live, you need to enter your cable subscription information. This goes for other channels as well, including Showtime Anytime, Lifetime, ESPN and History among others.
Other Apple TV apps (there are currently about 60 of them) require a stand alone subscription like Netflix, Hulu, HBO Now and MLB.tv. Apple's upcoming streaming TV service will also require a $30 - $40 per month subscription. Many apps are free, such as TED, PBS and YouTube, and you can stream content from anything you have on your iPad.