HID stands for high-intensity discharge. The bulbs inside HID grow lights are tubes filled with gas and electrodes on either end. Electricity jumps from one electrode to the other, reacting with the gas to generate bright light.
While HID technology is older than LEDs, these lights have higher light output and brightness. HIDs can also come in double-ended configurations, which are brighter and more space-efficient than single-ended lamps. Large commercial greenhouses usually prefer HIDs.