Luminance is a measure of how light or dark a colour is. A colour of any hue will approach a pure black as its luminance approaches zero or pure white as its luminance approaches 100%. This limits the saturation of any colour that has a low or high luminance.
Luminance refers to human perception of the colour quality, not to the power output of a light source. I can only make approximate adjustments to luminance settings when my program adjusts the output levels of the pixels on this computer screen.