You use resources to pay the numeric costs of cards and abilities. The more resources you have in play, the more things you can do during your turn.

If a card instructs you to play a resource, you have two options. The first is to put a resource card from your hand into play face-up. The second is to put any card from your hand into play face-down. Once a card becomes a face-down resource, you can still look at it later to see what it is as long as you control it. The advantage of playing actual resource cards instead of face-down resources is that resource cards give you resource icons.