Read the lyrics here and it pretty much explains itself. A lot of references to old S.F. Flicks and authors. Pretty much a joke song.
Rocky Horror Picture Show - Science Fiction/double ... - Lyrics
What's the song meaning of Science Fiction/Double Feature in Rocky Horror?
To ME . . . . It means fond memories of Saturday afternoons.
It's just a fun spoof about the old science fiction films. Theaters usually showed two movies for one price (and a cartoon). We called that a double feature :)