Brightness is a transparency type in the Transparency Tool. Depending on the color chosen, it makes the colors of underlying objects lighter or darker.

The color of the object with Brightness applied:

Gray (below a 50% tint of black) & White - makes the colors of underlying objects lighter. White has maximum effect

Gray (above a 50% tint of black) & Black - makes the colors of underlying objects darker. Black has maximum effect.

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