Have you ever played a card game that has wild cards? When you play with wild cards, playing a wild card automatically wins the trick for you. Sometimes wild cards can be jokers, deuces, or aces. It doesn’t matter what the card is; if you have one, you know that you have a sure winner. In Bridge, you have wild cards, too, called trump cards. However, in Bridge, the trump cards are really wild, because they change from hand to hand, depending on the bidding.
The bidding determines whether a hand will be played with trump cards or in a notrump contract (a hand with no trump cards). If the final bid happens to end in some suit as opposed to notrump, that suit becomes the trump suit for the hand. For example, suppose that the final bid is 4♠. This bid determines that spades are trump (or wild) for the entire hand.
More contracts are played at notrump than in any of the four suits. When the final bid ends in notrump, the highest card played in the suit that has been led wins the trick.