iOS 9 allows you to "shrink" a video you're watching into its own window on an iPad. This Picture in Picture (PiP) feature means that you can keep watching the film while you do other things. Here's how it works:
While watching video or using apps such as FaceTime and Videos on most iPads, press the Home button. The video reappears in a small window. (This doesn't work with all applications; for example, if you have the YouTube app on your iPad, it doesn't support PiP).
You can adjust the PiP window size. Pinch out with two fingers to enlarge the window or pinch in to shrink it.
To pause the PiP video, tap the PiP window and then tap the Pause button. To resume watching in fullscreen, tap the PiP window and then tap the Fullscreen button. To close PiP, tap the PiP window and then tap the Exit button.
You can drag the PiP window around the screen, but it will always snap to the edge of the screen. If you just want to listen to the audio, you can even drag the PiP off the edge of the screen. iOS replaces it with a small handle. To retrieve it, drag the handle.
By the way, PiP works only on the iPad mini 2, iPad Air, iPad Pro, and later models.