A die-cut card has had part of the card stock physically removed to create a specific shape, design, or function — like a stand-up display piece. It's cardboard engineering at its most creative.
In today's collecting era, die-cuts are typically used as inserts and are almost always short-printed, which makes them rarer than your standard cards in the set. That scarcity, combined with the unique visual appeal, means die-cuts are the kind of pulls that collectors actively chase. If you pull one, you've got something a little different from everything else in the box.