Resource Cards are a special type of drop that drop from most items with the "Depot" suffix at the end. Cards are all items in your inventory but Resource Cards specifically have an 80x80 image on a small box instead of the usual card's large image preview on it. Resources are used to purchase most items in the Shop.
There are many different resource cards that droppers will provide, but you can also get some from Alloying. Once collected they are placed in the inventory.
Here are all the current basic resource cards: