Since its inception, iOS devices have used a single home button navigating between apps. Click once to get back to the home screen, double-click to multitask. Case in point: Say you were in the Messages app and a Facebook notification required your attention. Tapping on the notification would result in the Facebook app launching, and nothing more.
If you wanted to get back to Messages, you'd need to press the Home button twice to bring up the multitasking interface, the tap on the Messages card.
With iOS 9, however, that process has been streamlined with a new back button of sorts.
As you can see in the screenshot above, when you switch to another app be it due to a notification or tapping on a link, the top status bar will now display a link that takes you back to where you were in the previous app.
I know this may seem minor, but after you begin using the link to go back, you soon realize just how useful it is. Gone are the days of double-clicking yourself between apps.