The main idea is this: the touchscreen can now tell light and hard taps apart, and then offer a layer of extra functionality with apps and throughout iOS 9. For the most part, however, this functionality is currently limited to just built-in apps, who will either allow you to 'peek' a level deeper (like preview an e-mail) or grant access to shortcuts if you hard press on their icon on your homescreen.
But that's only part of the promise of 3D Touch, the other being gaming. Everybody knows this: controls in many complex mobile games are a bit clunky and off-putting. With 3D Touch, however, game developers will have yet another layer to assign functionality to. Unfortunately, the list of games that already offer such functionality is still pretty short, with the majority of them simply having attached shortcuts to their icons — like to start a quick game in a specific mode, or some such.
Below, we've outlined a few games that are already making full use of 3D Touch. As you'll notice, the list is rather barebones, so we're guessing that the dozens of game studios are still working on making 3D Touch count instead of rushing to market with an under-cooked implementation. Let's hope we're right, for we need some more 3D Touch action, and now!