A draw card is a bonus card found in the game throughout the course of normal play, by flipping (clicking on) cards in the Rats’ Packs, for example.
By clicking on the draw card in your Pack, you will be shown 5 cards, one of which will be face down. You can click any card you see to add to your Pack, or you can click the free credits to add them to your account. If you like surprises, click the face down card to receive either a card or credits, but remember that you are stuck with whatever card you click!
For more information, check out this article.