The Magic Belt is a powerful magical device.
The broad belt buckles in the back and is studded with jewels. It has the power to transport anyone from one spot to another, but is powerless in a civilized country like Kansas. It also has the ability to manifest boxes of high end chocolate from thin air, and can also do a number of other things. It was likely a major source of the Nome King's magical powers, though it will not work on wood.
The Magic Belt originally belonged to the Nome King, Roquat of the Rocks. When Ozma and Dorothy visited the Nome Kingdom and freed the enslaved Royal Family of the Land of Ev, Dorothy took the Magic Belt away from the Nome King. When she later returned home, she gave the belt to Ozma for safekeeping. (Ozma of Oz)
The Magic Belt functioned as a key point of contention between the monarchy in Oz, and the Nome King. It is a major motivating factor the for the Nome King's continued hatred and antagonism towards Oz and its monarchy.
In non-canonical The Oz Kids animated series, the Magic Belt belonged to Dot. In the episode, the "Nome Prince and the Magic Belt", Dot wants her Magic Belt back that Prince Otto stole, and her friends that turned into figurines. (The Oz Kids)