The phrase refers to ensembles designed for two people that are inspired by the characters, aesthetics, and themes of the Game of Thrones television series and book saga, and are intended to be worn on Halloween. As an example, one individual might dress as Jon Snow, while the other dresses as Daenerys Targaryen, thereby creating a coordinated pair of costumes for the holiday.
The popularity stems from several factors: the wide cultural impact of the source material, the potential for detailed and recognizable character portrayals, and the inherent fun of collaborative costume design. Historically, fictional characters have long served as inspiration for Halloween attire; this particular trend reflects the specific dominance of Game of Thrones in recent popular culture.