Apple Watches shipped with watchOS 1.0, which supports third-party apps but requires them to push all of the heavy lifting to your iPhone. Apps that don’t run natively (so anything other than Apple’s built-in watch apps) couldn't tap into all of the watch’s hardware features, like the accelerometer, Taptic Engine, microphone, and heart rate sensor. That changed with watchOS 2. Apple just opened its features to app developers and let them test the watch’s boundaries.
Instead of waiting for the watch’s native Activity app to relay your heart rate to the iPhone Health app using HealthKit, third-party apps can run natively and collect that information on their own.
Ideally, this means we’ll see developers make use of features like the Taptic Engine in really interesting ways. Watch apps also run much faster, because native apps don’t have to wait for your iPhone to respond.