Will fix my entry as the following.
Credit tokens are essentially like coffers. During your turn, you may return N credit tokens to the supply, to gain a card from the supply costing exactly $N. You can do this whenever you are not resolving a played card. The longer you let your Credits build up,the bigger the potential reward - but don't wait too long! For example, suppose you have built up 6 Credit tokens. You could return 4 to the supply to gain a Smithy, or return 6 to gain a Gold. There's also the question of when exactly during your turn you should gain the card - maybe you can time it to get the card you really want at exactly the right time.
Note that you can't return 0 Credit tokens to the supply to gain a card, so you can't use that to empty the Coppers pile (or things like Wayfarer or Destrier, if they cost 0).
Gaining a card via Credits does not count as a buy.
Credit Note (Treasure, $5)
+$2
+1 Buy
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While this is in play, when you buy a card, +1 Credit.