Resources are cards that are usually used to pay Structure costs, or let Mobs and Players use certain abilities. Other than that they usually have no use. They have no icon in the top-left corner. They're played on your Resource pile, besides the Enviroment.
When using a Resource card's resource, per example to play an Structure, you must tap it by rotating it 90 degrees to the right. You untap your Resources on your next Start Phase.