I'm not sure specifically as there could be many interpretations. It might be a story where the character Harry from the Harry Potter universe is somehow related to being the first Hokage from Naruto. Maybe it's a crossover fic that combines the magic of Harry Potter with the ninja world of Naruto in a really unique way.
Well, in this 'harry is the first hokage fanfiction', it could be that the author is playing with the idea of Harry's leadership qualities and how they would translate in the Naruto world as the first Hokage. Harry has shown great courage and the ability to lead in his own story. So perhaps this fanfic explores him bringing those traits to build the Hidden Leaf Village, or maybe he has some kind of magic - ninja power fusion that makes him a very different kind of first Hokage than in the original Naruto story.
The 'harry is the first hokage fanfiction' could be a really interesting take on two very popular fictional universes. It might start with Harry being transported to the Naruto world for some reason. Then, due to his knowledge of magic and his experiences in fighting dark forces in his own world, he decides to take on the role of the first Hokage. He could introduce new forms of combat or ways of governing the village that are influenced by his wizarding background. For example, he might use spells to enhance the security of the village or use his knowledge of magical creatures to develop new ninja techniques. It could also explore how the ninja in the village react to his strange powers and different way of thinking.