A player must follow suit if that player has cards of the leading suit in his hands. If not, he has the choice of playing a trump card to possibly win the game, or rough (waste) a different suit. It is also possible that the specific deal has no trump. In that case, any card other than the leading suit played has no value, in most trick-taking games. In some games such as Oh, hell, where the player may need to not get more tricks to win, playing cards other than the leading suit can be useful.
For example, consider the following hand: TRUMP: Diamond (♦) North: A♠ K♠ 4♥ 9♦ East: Q♣ 10♣ 7♠ 5♦ South: A♣ 3♥ 2♦ J♦ West: 5♠ 5♣ 6♥ 2♠
North leads the deal with K♠. Now, all the other players must follow suit, i.e. play a spade card. East has a spade card, and thus must follow suit by playing 7♠. South, however, does not have any spade card, and thus is allowed to play any card he wants. If he desires to win the deal, he can override North's K♠ by playing a diamond card (diamond being the trump), for example J♦. If he does not want to win the deal, he can sluff any other suit, such as 3♥. Let us assume that he plays J♦, overriding North's card. Now, West still has to follow suit, since he has a spade card, and plays 2♠. South's trump card, gives him an opportunity to escape following suit, and he wins the deal.