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.
It can change the whole mood of the story. If Y N gets hurt, it might introduce a sense of urgency and concern into what was previously a happy and exciting concert - going experience.
Perhaps Y N's injury is not too serious at first, but it starts to get worse as the concert goes on. In the fanfiction, this could lead to a very emotional moment where Y N is taken backstage and the EXO members visit them to offer their support and concern. It might even start a friendship or a special connection between Y N and the group.