Game-worn memorabilia is gear that a player actually wore during an official game — and then a piece of it got embedded into a trading card as a relic. We're talking game-worn jerseys, gloves, pants, and other uniform pieces.
The distinction between game-worn and event-worn matters in the hobby. Game-worn means the item was used in actual competition, which adds authenticity and value. Event-worn memorabilia (from signings, photo shoots, etc.) is still cool, but it doesn't carry the same weight.
When you're holding a relic card with game-worn material, you're holding something that was on the field during a real game. That connection to the actual moment of play is what makes game-worn relics so special.