On this page of the guide to Aliens: Fireteam Elite you'll learn more about the Challenge Cards. The cards, some of which make the game easier (although most of them increase the challenge) offer rewards for fulfilling the objective on the card. You'll learn how to earn these cards, as all of them can be used up.
Challenge Cards are utility cards earned while playing the game. You can obtain them for finishing daily missions. It's also possible to buy specific Challenge Cards from the trader.
The effects of the cards vary wildly. Some Challenge Cards help you beat a mission but don't offer any additional currency multipliers. There are also cards with an opposite effect. They give you a difficult goal, make enemies stronger/reduce your resources etc. Some cards completely change what the player sees on the screen.
Each of the cards should feature some writing informing you about an additional multiplier, if it's not there, it means the card doesn't increase your money gain. It's worth mentioning that three players can use their cards, but ultimately the game will choose only one of them at random.