Output is your total score for the workout and for each round. It's what you should aim to increase over time / every time you take a class.
It's based on the number of punches you throw, the speed of each punch and the technique. For 2 punches that both reach the same speed, the one that was quicker / tighter (less time to reach the speed and land) will get a higher score.
Hit hard, hit often and you'll crush your previous output!