Resources are the glowing blue icon just to the left of your cards.
At the beginning of each new round you earn 3 resources. Any resources you don't use carry forward into the next round.
Every time you play a new card it will cost you resources. Each card has its own cost, eg the Gandalf card costs 5 resources to play where as the Self- Preservation card costs just 1 resource to play.
To earn this achievement you will need to spend 100 resources. Therefore you will quickly earn this by just playing the game however to speed up the process simply play the higher value cards more often.