I'm not sure specifically as I haven't read that exact 'Naruto' fanfic. But generally, it could be about a dangerous or harmful element (the 'drop of poison') within the 'Naruto' universe. It might involve a new villain or a dark plot that threatens the characters we know and love in the 'Naruto' world.
Since it's a 'Naruto' fanfic, 'a drop of poison' might be a metaphorical or literal device used to drive the story. It could be that there is a political or social 'poison' seeping into the Hidden Villages. For example, a traitor within the village might be like a drop of poison, slowly corroding the unity and safety of the place. The fanfic could then explore how the 'Naruto' characters, with their unique abilities and values, try to combat this 'poison'.