It means the same as to trump in whist, or in any card game which involves a suit being chosen as trumps.
When a player plays the first card of a new trick, all other players must follow suit, ie play a card of the same suit as the first card. But if a player does not hold any cards of that suit in his hand he may play a card of any other suit.
Before the game starts, one of the suits will have been chosen as trumps. If the trump suit is not being played in a given trick, a player not holding a card of the suit that is being played may play a card from the trump suit, thereby 'trumping' and so taking the trick (regardless of how high the value of the highest card of the suit played, and regardless of how low the value of the trump card).