Game contribution is a rule that determines how much of your stake goes towards the wagering requirements depending on the game you play. Online casinos will lower the game contribution of high odds games to lower their risk.
In most cases, slots will contribute 100% to the wagering requirements. Therefore, if you wager €100 on a slot game, the entire €100 will go towards the wagering requirements.
Blackjack, on the other hand, may have a game contribution of 6%. Therefore, if you wager the same €100 on blackjack, only €6 will actually contribute to the wagering requirements.