Double-clicking the Home button shows you all the apps that are running on your iPad in a bar at the bottom of the screen. To switch to a running app just tap on it. When done swipe the screen downwards to remove the bar.
To close running apps, double-click the Home button and then Tap and hold down on any one of the Apps that is currently running. You’ll notice that each App will start to wiggle and now has a minus sign. Tap the minus sign of the Apps that you want to close.