The trump card is the winning card. If you play a trump card during a game of Spades, it means that you take the whole trick — in other words, you'll pick up every card on the table.
Any card game in which one card or suit ranks the highest has a trump card, the card that wins a hand. You can also use the phrase metaphorically, to describe some secret advantage you have. For example, you might think of your mom's homemade brownies as your trump card when you're trying to convince your friends to come over after school. Trump card comes from an old card game called "triumph," shortened to just trump in the 1500's.