Until now, Apple has had very stringent rules about how apps can run in iOS. Apps have always run in "sandboxes" so they can't talk to each other. Apple is finally loosening this restriction so that apps can work together.
In short, Apple is expanding its support of extensibility, so that apps will be able to offer services to other apps. The extensions still live within sandboxes, but other apps will be able to reach in. For example Pinterest will be able to offer sharing to the system, or you you might be able to use VSCO filters directly from the photos app.
Admittedly, Apple is being a little vague about how this extensibility might work, but it's a pretty dramatic shift in posture from before.
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