Lighten is a transparency type in the Transparency Tool. It is useful for making underlying objects lighter. 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). Lighten is a grayscale version of Bleach transparency type.

The color of the object with Lighten applied:

Gray & White - lightens the colors of underlying objects.

Black - no effect on the colors of underlying objects.

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