Saturation is a transparency type in the Transparency Tool. Usually you would use it on a separate object to modify the color of underlaying objects (that is, you would use an object as a filter). Depending on the color chosen, it makes the colors of underlying objects brighter or duller.

The color of the object with Saturation applied:

Gray (below a 50% tint of black) & White - increases saturation in the colors of underlying objects towards pure color.

Gray (above a 50% tint of black) & Black - reduces saturation (colorization) towards black.

Other colors - convert the color to the equivalent shade of gray. (Yellow gives light gray, Red dark gray.) Then increase or decrease the color saturation of underlying objects.