"Hey Siri" is a new feature in iOS 8 that makes it possible to invoke Siri using only your voice. It's nice for those moments when your hands are full or when your iPhone is across the room.
The only downside to "Hey Siri" is that your iPhone must be connected to a power source in order to use it. This restriction was placed there to save your iPhone's battery life.
But, there is a way to use Hey Siri without being connected to a power source. It's not as handy as using it while connected to power, but it's a nice workaround for those situations where you need hands-free control.