When you play this, you may play a Treasure from your hand twice. If you do, trash that Treasure.
Counterfeit is a Treasure card from Dark Ages. It functions as a Throne Room variant and trasher for Treasures, allowing you to play another Treasure card twice while trashing it. Early in the game, Counterfeit can function like Moneylender, allowing you to trash Copper for a total of +3 Coin; in the endgame, it acts more like a trash-for-benefit card, allowing you to trash higher-value Treasures such as Gold for large amounts of payload.