When you buy this or play it, each other player reveals the top 2 cards of his deck, trashes a revealed Silver or Gold you choose, and discards the rest. If he didn't reveal a Treasure, he gains a Copper. You gain the trashed cards.
If this is the first time you played a Fool's Gold this turn, this is worth 1 Treasure, otherwise it's worth 4 Treasure.
When another player gains a Province, you may trash this from your hand. If you do, gain a Gold, putting it on your deck.