Color saturation refers to the intensity of color in an image. In technical terms, it is the expression of the bandwidth of light from a source. The term hue refers to the color of the image itself, while saturation describes the intensity (purity) of that hue. When color is fully saturated, the color is considered in purest (truest) version. Primary colors red, blue and yellow are considered the truest version of color as they are fully saturated.
As the saturation increases, the colors appear to be more pure. As the saturation decreases, the colors appear to be more washed-out or pale.