Honestly, 'y f n' in fanfic could be a random combination of letters without a widely recognized meaning. Maybe it's a made-up term by the author for a special plot element or character trait.
I'm not entirely sure specifically what that particular 'bts y n is a hybrid hurt fanfiction' is about without more context. It could be a fan - created story that involves the BTS members, some kind of 'y n' (which might be a made - up or specific character in the fanfic world), and it's a 'hybrid' concept which could mean a mix of different elements like human and something else, perhaps supernatural, and 'hurt' might imply there are elements of pain or emotional distress in the story.