The letter was written by Percy's friend, Annabeth Chase, and explained that Percy was framed for stealing Zeus's lightning bolt and that he was on a quest to clear his name. The letter also included a map showing Percy's whereabouts and a request for the gods to help him.
This letter was crucial to Percy's quest as it made the gods aware of his situation and started the chain of events that would eventually lead to him finding the real culprit and clearing his name.