In Balatro, Red Card is a Joker that gains power whenever you skip Booster Packs. Specifically, it adds +3 Mult each time you skip a pack after opening it. This means you must purchase a Booster Pack and then choose not to take any of the offered cards for the effect to trigger. For Mega Booster Packs, which offer two choices, you can skip one of the two cards to activate Red Card, but the bonus can only be triggered once per pack, not per card.