Background lighting is when the main source of light comes from the back of the subject not from the front. Background lights are usually placed on a stand between the background and the subject. If the light is adjusted properly it will provide good tonal separation between the subject and the background.
The intensity of the background light shouldn't be more than main light falling on the person's face. If you increase or decrease the intensity of background light, you can control the tone or color reproduction of the background in the finished product.
If you want to reproduce the background color to its true color in a color photo, it has to get the same amount of light as the person's face. It's better to set up a background light before any other light since it's easier to determine its effect without any interference from the main and fill lights. A background light should be placed so the brightest part of the light illuminates the background right behind the person's head and gradually falls to the corners of the image. This will separate the person's head from their body and will draw attention to their face.
Various types of lights can be used for background lighting and can range in price from about $10 to several thousand.