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.
If you complete every single repetition of a Workout or Exercise session with perfect form and perfect range of motion according to the instruction videos, you can earn a Star for your performance. When you finish your training, you will be asked to rate your technique for that particular session. If your technique was good, your rating should reflect this and you will then earn a Star. Keep in mind that when you rate your trainings with a Star, this will impact your next Coach Weeks. Star-rated trainings also reward you with extra points.
Sometimes you might feel that you need to do the easier version of an exercise to finish your Workout. Just remember to only give yourself a Star when you perform the exercises exactly as the Coach has planned. Regardless, always try to execute all exercises with perfect form. Focusing on movement quality makes all the exercises more effective and is thus always better for you and your progress.