what is the untethered boot?

Well, iPhone tethered boot is when you hack an apple device, some of the available jailbreak software requires you to be connected to a computer each time the iPhone device is reset. The untethered boot does not need to be connected  to a PC to reboot.

This does not include when an iPhone device goes to sleep. This becomes a problem when you are out of battery as you need your computer to turn your phone on. In the case of the AppleTV it doesn’t really need batteries so it should require a reboot less often. In my experience, AppleTV does require a reboot once in a while though as it can lose connectivity to the network.

With this in mind there are software solutions that specialise in untethered boots. Greenpois0n is one of those and that is why I have used them with my setup.