Monkey D. Luffy's straw hat is the main symbol of the entire series and is the origin of his nickname "Straw Hat Luffy." His straw hat was given to him by his idol, Red Hair Shanks when Luffy was a child, and he promised to give the hat back to Shanks once he became a great pirate (or in other words, the Pirate King).
Grab his straw hat today