During an offline workout the double tap will add a placemarker in the workout history. For example you can double tap when you start the workout, end the workout, or to show breaks or lap splits. This feature is only available in the iOS Wahoo Fitness app.
During a workout with your phone the double tap will control your music playback by default. However you can edit this to perform other functions under the workout settings (Settings > Workout Profile of your choice > Double Tap).