Master thrall was a rank within the Providers' thrall system. The master thrall was in control of other thralls' collars of obedience. When activating other thralls' collars, the master thrall's eyes would glow. Master thralls also wore collars of obedience which were controlled by the Providers themselves.
Galt was a master thrall in 2268. His collar, unique among those seen, bore the colors of all three providers, possibly an indication of his special status within the thrall community. (TOS: "The Gamesters of Triskelion")