The Glow in the Dark Undies are a pair of briefs fitted with a sheath to accommodate a man's member. The idea is that one will wear them in the dark with an erection and...well, as the box copy puts it, "Make it glow for a great show!"
The sheath will accommodate - uncomfortably, I'm told - an 8-inch cock, but will do so just barely. I was amused despite myself by the figure my friend cut in his new briefs when the lights went out, but then I realized quickly that there are better things to do in the dark with an erection and two consenting adults than ponder the wonders of glow-in-the-dark drawers.