iOS has long had a way to switch between apps, but it’s a tedious process of double-tapping the home button and tapping the previous app you need. It’s one reason why many apps that deal with web content built in their own, basic browser so you don’t have to bounce back and forth.
iOS 9 introduces a great new, one-tap way to get back to your previous app. In my example here, I tapped a link in a Mail message which opened Safari. But now there’s a new button in the top left of Safari to go “Back to Mail.” One tap and I’m back. Excellent.
I believe this is a feature that developers have to add to their apps to allow this behavior, but I’m not sure.