While you’re using an app, you can switch it to a Slide Over window—a smaller window that slides in front of another app or window—and open another app behind it. For example, while you’re using the Photos app, you can have Mail open in a Slide Over window and create an email message while viewing photos.
You can have multiple apps open in Slide Over. iPad keeps track of the apps you open in Slide Over so that you can switch between them easily.
Tap to place the current app on the right.
On Switch between open apps