After you've defeated an enemy you are rewarded with loot. This can include gold, relics, cards and potions. Do not feel like you have to pick everything up. The most important things is to skip cards. Most successful runs are because you have a solid and consistent deck. When you get to pick between 3 cards and you feel that none of them fit your deck, then don't pick them. You'd rather want a small deck with good cards that synergize with each other, rather than having a big deck with amazing cards that doesn't synergizes. And also, the smaller the deck, the more consistency in each turn of combat.