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.
Tap on one of the icons to launch the app in slide-over. Notice you can then use the app as you would if you were using the app in fullscreen mode. When you're finished, just slide the left-edge of the slide-over panel to the right to close it or tap outside of the panel.
Currently, only Apple apps are working with the new feature. However, once iOS 9 is officially released, developers will have the ability to release updates to apps, adding slide-over compatibility.
Which leads to one big problem with the feature: You cannot disable or remove apps from showing up in slide-over, meaning the column of apps is going to get rather crowded. Hopefully Apple adds the ability to remove apps (or developers include the option within their own apps).