The drain valve is used to let the water drain out of the water heater. Many of the quality water heaters are constructed with a brass drain valve which is much easier to service for maintenance. In order to save on costs there are several water heater manufacturers that use plastic drain valves such as in the image above. A plastic drain valve can be difficult to operate if it has been sitting unused for a length of time.
It is very common for a drain valve to get clogged with sediment and once opened will not allow for the water to drain out. In some cases the easiest way to drain out the water heater is to completely remove the drain valve. Most drain valves will unscrew with a wrench by turning it to the left.