I found this during my recent walk through a home. My client asked what that was. Oddly enough some one else asked me about them recently too.
This is a UV Filter. They attach to the duct-work of a furnace. Basically as the air blows past it, the UV filter kills of organisms by hitting it with UV rays. This one simply plugs in but there are many types.
From the Swordfish web site “Most indoor air quality products merely trap pollutants, but Swordfish actually kills molds, viruses, and fungus in your indoor air. Swordfish UV Air Treatment systems use proven technology and expose your recirculated indoor air to UVC light which kills/deactivates micro-organisms and sanitizes your air.”
You still need a normal mechanical filter.