Event-worn memorabilia is gear that a player wore at a non-game event — think signing sessions, photo shoots, promotional appearances, or rookie showcases. The material gets embedded into relic cards just like game-worn pieces, but the provenance is different.
The key distinction: event-worn items weren't used in actual competition. A jersey patch from a rookie's signing day is event-worn, not game-worn. Both end up in trading cards, but game-worn memorabilia generally carries more collector appeal and a higher price.
If the difference matters to you (and for many collectors it does), check the card's description carefully. Topps and other manufacturers typically specify whether the relic is game-used, game-worn, or event-worn.