What is "One piece" about?
The main character "Luffy" made a promise to a man named "Shanks" that he'll become pirate king. Luffy ate an "Devil Fruit" which man him into a "Rubber Man" letting him to stretch his body.
Its about pirates fighting to find Gold D. Rogers treasure on the grand line and become the pirate King
Along the journey, he searches for the ultimate treasure, "One Piece" with the help of his friends.