The HD is their new line of LED lights, released maybe the last 3 to 4 months. According to Reefbuilders:
But the really exciting change to the Hydra HD is a new LED driving system called HyperDrive which lets users get the most from the LED channels they are using with the Hydra HD. Nearly all LED lights have a maximum power output, and a maximum power level for each channel.
With HyperDrive the Hydra 26 and Hydra 52 can redistribute the available power so you can "overdrive" the LED colors that you actually want to use. What HyperDrive essentially means is that now if you have a Hydra HD, you';ll be able to not only turn down the white, green, and red channels of color, but also turn up the power on the blue, royal blue and UV channels.