Some cards call for you to pay Coins in their ability text. In order to use the ability, you must pay the indicated number of Coins immediately. Coins spent in this way are, of course, lost. Also, you need to have the Coins to pay before you play a card whose ability costs Coins. If you are unable to pay the required number of Coins, the card’s effects after that are ignored and do not take effect.
Example: You have a City Development card and 2 Farming Village cards. You first play one Farming Village card to get a Coin. After that, you play the City Development and pay a Coin, which lets you banish the remaining Farming Village in your hand to get a City card from the Market.