The battle pass in Fortnite it's a kind of membership to enjoy some different game experiences and quests, with the chance to get rewards like V-Bucks, the in-game currency, and unique items not found in the in-game store.
Many wonder if this pass is unique and does not end. But the answer is no: because the game is constantly updated and there is a unique and different battle pass for each season of the game. So you must hurry up and complete all the challenges of the battle pass before the end of the current season. Therefore, the battle pass lasts for the duration of the game season.