The YAML file is a Deck object containing a series of sub items, the text of those subitems will become the descriptions of the cards, and the name of the subitems becomes the titles of the cards (obviously, see the example).
Placing png files in the Backgrounds directory with their file name set to the card's title, (so, a card called Sword's background file is Sword.png), will make that png file centered in the middle of the card (allowing card art).