Think of it this way. A lot of times links within apps take you to the tiny, mobile version of a website even though the app experience is so much better. For example, if someone sends you a link to a Spotify playlist, wouldn't it be better for it to launch straight into the Spotify app, not the website?
Facebook's App Link allows developers to program their apps to do just that. In short, developers will be able to easily let apps talk to each other, so you're not constantly closing out of one app to access related content in another.
It's a big move that fixes a problem with many mobile devices, especially iPhones and iPads. Apple doesn't let you choose default apps for stuff like maps and Web browsing. You have to use Apple's defaults apps for those functions. Facebook's App Link will give developers (and you) the power to quickly jump between the apps you want to use.
App Link also works across all the major mobile platforms - iOS, Android, and Windows. That means users won't have any problems no matter what device they're using. In a way, Facebook is using App Links to take control over the way apps talk to each other on smartphones and tablets. Apple, Google, and Microsoft haven't made that very easy for developers, so Facebook did it for them.
In the end, it's going to be better for you. Facebook now has control over the way apps and content within apps talk to each other, but you're going to get a richer and smoother experience on your phone.
App Link also plays on a theme a lot of smart folks in the mobile industry have been talking about lately. People use apps on their phones more than mobile websites, but so far no one has figured out how to be "the Google of mobile apps." That is, no one has figured out a good way to make your phone's apps talk to each other yet and help users manage content between their apps.
With App Link, Facebook seems to have done just that. And the future implications of that could be huge. Mobile apps are the future of computing, and App Link will give Facebook a lot of control over a lot of what we do on our smartphones, even if they're not directly tied to Facebook.
Basically, Facebook found a way to take over your phone.
Facebook already has a bunch of partners using App Link starting today, including Tumblr, Pinterest, and the popular email app Mailbox. Plus, App Link is open for any developer to use, so expect the list of compatible apps and websites to grow.