A headmounted display (HMD) from Sony, likely an early virtual reality or augmented reality device. In the context of the conversation, it was a device John Carmack was trying to modify to reduce latency.
John Carmarmac started posting on that same forum asking for help modifying his own Sony headmounted display that he had bought to reduce the latency.
""The speaker mentioned John Carmack attempting to modify his Sony headmounted display to reduce latency, indicating it as a specific, albeit older, piece of technology."