Challenge mode: the video plays at your speed, allowing you to perform the same overall effort as the video maker. If you're not fast enough, the video slowly decreases the framerate to finally wait for you to catch up if you're around 50 meters behind. If you're performing well, the framerate can be increased up to twice the original speed. Use this mode to compete in challenges like the Indoor Challenges, and to have export to 3rd party websites such as Strava with full coordinates and map of your virtual ride.
Discovery mode: the video plays at the original speed, pausing when your speed is reaching 0. Whatever your performance is, the video framerate is not altered. But you'll still see if you perform well by looking at the energy output produced, measured in Kinos. Note that export to 3rd party websites such as Strava will be limited to Watts and any additional data like cadence, heart rate, but will not display coordinates nor a map.