When a fire stick is jailbroken it simply means that the fire stick has had KODI media server software installed on the fire stick to Circumvent Itunes Digital Rights Management on the Movies, TV programs and Music that is being watched or listened to on the Fire Stick.
The KODI Software is media player software that is free for anyone to download and is legal but things can get messy if you actually use the Fire Stick with the software downloaded on it if some company finds out about it.