The king has a hat that's made from a tree but the more you look at it, the more it looks like it's just a big cork. It's a big, wooden plug that he's just placed on his head. It can hardly be called a hat.
It turned out this cork hat was given to him by a centaur (せん). You look into it and it turned out it IS a really big cork that centaurs use on their big wine casks. But the king didn't understand, and thought it was a hat. The centaurs are probably still laughing.