These line the streets leading to the "Man" along three cardinal directions, and line the interior of the residential areas (i.e. the semicircle closest to the Man). Lighting them at dusk is done as part of a bit of a ceremony, with the people involved wearing white robes.
The point is that volunteering to be one of the "lamplighters" for an evening is one of the easiest ways to volunteer at Burning Man, and in my opinion doing at least one volunteer activity, on at least one day, is an essential part of feeling like a part of the event.
