Slide-over is a new feature you’ll find on your iPad shortly after updating to iOS 9. In a bid to increase the multitasking capabilities of its tablet lineup, Apple is adding features that make it possible to do more than one thing on your iPad at a time.

Accessing slide-over is simple; when using an app, slide your finger in from the right edge of your iPad’s screen. A small panel will slide out along with your finger, revealing a column of app icon. In the event an app is already open, you can slide your finger down from the top of the slide-over panel to bring up the list of apps.
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https://www.cnet.com/how-to/how-to-use-the-ipads-new-slide-over-feature-in-ios-9/