With iOS 13, Apple added a Picture in Picture Mode to the iPad, and with iOS 14, that Picture in Picture functionality is available for the iPhone too, letting users do things like watch videos and take FaceTime calls while doing other things on their device.
Picture-in-picture (PiP) is a feature of some television receivers and similar devices. One program (channel) is displayed on the full TV screen at the same time as one or more other programs are displayed in inset windows. Sound is usually from the main program only.
Picture-in-picture requires two independent tuners or signal sources to supply the large and the small picture. Two-tuner PiP TVs have a second tuner built in, but a single-tuner PiP TV requires an external signal source, which may be an external tuner, videocassette recorder, DVD player, or a cable box. Picture-in-picture is often used to watch one program while waiting for another to start, or advertisements to finish.