Luffy’s straw hat is the main symbol of the One Piece series and is the origin of his nickname “Straw Hat Luffy.” Red Hair Shanks gave it to Luffy when he was a child, and he promised to give the hat back to Shanks once he becomes a great pirate.
Much later, during the Straw Hat Pirates’ departure for Fish-Man Island, it was revealed by Silvers Rayleigh that this straw hat originally belonged to Gol D. Roger.
Because the straw hat is so important to Luffy as well as the entire series, I don’t think it’s just a normal hat. I was thinking about the meaning of D, and found out about the Dawn Theory. I noticed something about the shape of the straw hat, it looks like a sun rising from upfront (the sun is the top part, and the horizon is the circumference of the hat).
Thus, I think the straw hat is the crown of the D clan/family, and it was passed on whom might have inherited a special will (It may be Joy Boy in this case, if he was really the king of the Ancient Kingdom).
Also, the second ship of the Straw Hat Pirates is called Thousand Sunny. Romance Dawn was the name of One Piece before it became One Piece. And if it’s ever confirmed that D stands for Dawn, and the straw hat is a symbol of the sun rising, it makes so much sense.