The heat sink is a material in the LED system that absorbs and disperses heat produced by an LED. The material is typically a metal, most often aluminum, but can sometimes be made from a durable polymer or plastic. Aluminum has become the industry standard for heat sink material due to its high thermal conductive qualities and ability to mold. Other metals such as steel or copper might be found on very rare occasions.
The purpose of a heat sink is to spread the heat out and dissipate that heat into the air. These take many shapes and forms, but are typically used in a fin-like array on most LED grow light systems. Without a heat sink, the LED will overheat and fail quickly.