A radio trailer is an audio advertisement used to promote a film without visuals. It aims to inform listeners about the film and encourage them to watch it through revealing the title, rating, and brief voiceovers from the script to convey mystery. Successful radio trailers are 30-40 seconds, include conventions like the film slogan, actor names, sound effects, and the director to engage listeners and depict the genre of the film. The goal is to pique interest and suggest where and when the film can be viewed.