This is a relative description of how much light is coming from the color. If the color reflects a lot of light, we would say that it is bright. Imagine seeing a red sportscar during the day. Its color looks bright. Compare this with the perception of the car as night is falling.
Less light means the color looks darker.
In the GIMP, the most important measure of brightness is measured by a quantity called value. However, there are also other measures of brightness that will be introduced shortly. For the moment, though, the horizontal bar in Figure 5.3 illustrates a range of red values. Value, like saturation, is a percentage that goes from 0 to 100. This range can be thought of as the amount of light illuminating a color.