Anything that has a real person (celebrity) in the story as themselves. ‘Celebrity’ fanfiction includes sports stars, actors, singers, youtubers and anyone else in the public eye. This doesn’t include incidental appearances. For example - if in your story a girl goes with her friend to a concert featuring BTS or Shinee and gets autographs from the guys, but then you never see them again, it’s not Fanfiction. If she starts dating Taeyong or Minho, then it’s become Fanfiction.
If you use a world or character from a published story, movie, TV show, it’s Fanfiction. Even if you use you and your friends as the characters, if you all go to Rivendell or Hogwarts, it’s Fanfiction. That includes anime, manga and video games. Anything that includes characters or setting from Voltron, Five Nights at Freddy’s (or Skyrim) is Fanfiction.