This is determined by the amount of light a color reflects. When we are talking about pixels we can also refer to the luminance value of a pixel's color. Luminance has meaning in both pigment and light color theory. In pigment color theory, luminance refers to the amount of reflected light. The Source l uminance option works with the luminance value of the colors in an image.
Lets take a look at a basic example.
In the following example, the original image was split into RGB channels. Then two copies were created. In each image a channel was loaded as a Mask using the Source l uminance option and each mask was inverted. In Example One, the Red channel was loaded. In Example Two, the Green channel was loaded, and in Example Three the Blue channel was loaded.