A light baffle is simply an opening through a box (which could be a wall cavity) that allows air flow but that prevents light from coming through.
The basic principle of a light baffle is that light (and we won't get into the hoary old argument of whether light is a ray or a particle) follows straight lines - except when it reflects or refracts. Light baffles don't involve glass, so we don't have to worry about refraction.
The sketch illustrates what the cross-section of a light baffle might look like. Note that it's simply a couple of barriers that keep the light entering one hole from passing straight through to the other hole. The interior of the baffle is painted flat black to prevent light from entering one hole and bouncing around the baffles to exit the second hole.