The Runkeeper app is an Apple Watch app that allows you to track your runs and workouts without having to take your phone with you. A mobile version of the app was previously available, but this version allows you to enjoy it on your smart watch.
When you open up the Runkeeper app on your Apple Watch, you are asked what type of workout you want to start. Once you reveal that information, you give the app permission to access data stored in your iPhone's central Health app.
At that point, you're good to go -- your smartwatch Runkeeper app is now on top of all your runs and workouts and will help you take your performance to the next level.