HPS lighting (high pressure sodium) is one of the traditional ways to provide light for plants to grow, often used in conjunction with MH (metal halide) lights.
Light is one of the most important factors (along with nutrients and water) for growing plants. And when plants are grown indoors, it is the one you will need to consider most, particularly if you need to rely on artificial lighting.
Plants do not need the full spectrum of light to grow. During early growth stage, plants need blue light while orange/red light becomes more important during the reproductive stage.
HPS grow lamps produce a yellow light and they’re typically used in the second, reproductive stage of plant growth.
HPS has several benefits:
The bulbs emit a lot of heat. This can cause “leggier” growth in your plants. To reduce heat, you will need to make use of a cooling system such as a fan.
A lighting ballast is a piece of equipment required to control the voltage of electrical discharge lights when they start and also during normal operation. If you are familiar with fluorescent tubes (strip lights), the ballast is the little "starter" component that looks like a little battery. You will need a ballast to run your HPS system.
As HPS grow lights emit a yellow light suitable for the reproductive phase, they are often supplemented with metal-halide grow lights which produce the blue light required in the growth stage.