The level of each exercise is automatically adjusted to your performance and increases with your training progress. The higher the level of an exercise, the higher the difficulty of that exercise.
Adjust exercise level
Should you wish to manually adjust the level of the exercise, do so by clicking on the blue slider and sliding it to the right to increase the level of the exercise, or to the left to decrease the level.
The grey lock you see on the right prevents you from setting the level too high. As your exercise level progresses, the more levels you will unlock and the higher they can be manually selected.