In this very bizarre short story, Elllison seems to be using the old saying of "like a moth to a flame" to parallel his main character (who is named Moth, of course), who is drawn to experience the misery of every single passenger of a nameless vessel travelling through something called the Megaflow. Every passenger on the ship is a stranger to each other (and possibly themselves), and no one talks to anyone except for Moth, who becomes their secretly held miseries, drawing them out.
It seems to me that the ship is some sort of bleak metaphor for life and the miseries we all come to experience. Moth himself, through this metaphor, and through his ability to let people displace their miseries on someone else; to vent their self-loathing, becomes an immensely tragic figure. I say tragic because each stranger on the vessel is allowed to leave. However, Moth, who is a permanent resident of the vessel, must stay on, presumably to continue absorbing the miseries and self-loathing of each passenger. Essentially, it shows that, no matter how bad your own individual miseries are, they are better than having to experience every collective misery.