When you perform a Workout, Exercise, or Interval, you will earn points. Depending on the length and the difficulty of the Workout, the number of points changes. All Intervals, however, are worth the same number of points, as they are designed specifically for you and your personal goals. Thus, they are all equally important for your personal progress.
Over time, as you complete more and more trainings, your total number of points will increase. This is reflected in your personal Level. But try not to rush things, you have all the time you need to progress from one level to the next.
The Star is meant to signal that you have completed a workout exactly how it was assigned. If you complete every single repetition of a Workout or Exercise session without adapting the assigned exercises, you will be automatically rewarded with a Star. Star-rated trainings are rated higher than non-Star performances and will award you with extra points.