A jersey card is a type of relic card that features a swatch of jersey fabric embedded directly in the card. You're literally holding a piece of a player's uniform — the same material that was on the field or at least on the player's back during an official event.
Jersey cards are one of the most common types of relic cards, and they come in all tiers — from single-swatch base relics to multi-swatch premium versions with larger, more colorful pieces. The better the swatch (multi-color, nameplate, logo), the more valuable the card.
It's worth noting that not all jersey cards contain game-used material — some feature event-worn or player-worn swatches. Always check the card's description to know exactly what you've got. Either way, jersey cards add a tangible, physical connection to the game that standard cards can't match.