Players of 21 or younger can have bonuses to all of their skills during a match. This reflects the enthousiasm but unpredictability of young players. The younger a player, the higher the bonus he can receive.
The bonus consists of a boost to the speed, power, blocking, dueling, passing and scoring skills. Tactics is not boosted. Also, if a player has a high match bonus, his endurance will be lowered resulting in a higher fatigue.
The maximum boost for these skills for your player depends only on his age. You can find this value on the overview page of your player. The bonus for a match is determined once at the start of a match and is the same for all skills. If the bonus is set to 0.5 ball for example, the player will receive +0.5 ball on ALL his skills.
The dark grey star value represents the value your player would have if he were to receive the maximum bonus during a match. If, for example, the maximum bonus is 1 ball then the dark grey stars show the value of your player if he would have 1 ball bonus on all of the above mentioned skills.
This is therefore a measurement of current maximum potential, not of future possible potential, though players with a low silver star value but high grey star value probably have a promising future.