That 18-20 represents the level where a Star potential player would cap, and stop improving even if you tried to train them.
The column to the left of that 18-20 example is what the skills entered in the training planner calculate as applying to the cap. So if you have a star potential player, and you enter a set of skills you want to try and reach with your training, that number has to be lower than the player's potential, or else you might cap the player before he reaches your target.