A card binder is a special book that people use to keep their cards safe and organized. It's like a big notebook with plastic pockets inside where you can slide your cards in. It's like having a big folder with lots of plastic sleeves inside!
People who collect trading cards or sports cards like to use a card binder to keep their cards in good condition. If they just kept the cards loose, they might get bent or ruined. But if they put them in the binder, they are protected and can be flipped through to see all the cards easily.
It's also a good way to keep track of which cards you have and which ones you still need to find. Plus, it's fun to look through the binder and see all your cool cards!