Picture in Picture mode is a type of multitasking that was introduced with iOS 9 so you could complete other tasks while simultaneously watching video. Unfortunately those with iPhones, iPod touches, and even some models of iPad can’t use it because it's restricted to certain models of iPad only.
A new jailbreak tweak called ForceInPicture is now available in Cydia that will enable Picture in Picture mode on all unsupported devices, so long as you’re running iOS 9 or iOS 10 on them.