A relic card has a piece of actual memorabilia embedded right in the card — a swatch of jersey fabric, a chunk of game-used bat, a piece of baseball leather, you name it. You're holding a card with a physical piece of the game built into it.
Relic cards come in different tiers too. Dual relics feature two pieces of memorabilia, and triple relics pack three onto a single card. The more memorabilia, the more premium the card.
Relics are classified as chase cards and carry significantly more value than standard base cards. They're the kind of pulls that make you pause and appreciate the fact that a tiny piece of a real game or a real player's uniform is now sitting in your collection.