Picture in Picture mode allows you to swipe up/over on a full screen video to minimize it into a smaller window on your Home screen. This allows you to continue watching a video while accessing the apps or widgets located on your iPhone.
As with all new iOS features, Picture in Picture will be useful to some iPhone owners and a nuisance to others. Those who do not enjoy the feature can navigate to Settings > General > Picture in Picture. Here you will find one option, a Start PiP Automatically slider. Tap this slider to disable the swipe up/over option to minimize full screen videos. You can still invoke Picture in Picture by hitting the PiP button on a full screen video.