Well, 'Longlegs' being based on a true story means that there are real elements within it. Maybe it was inspired by someone's actual journey or struggle. For instance, if it's a story about a long - legged athlete, perhaps there was a real athlete whose story was tweaked a bit to create the plot of 'Longlegs'. It gives the story a deeper meaning as it has roots in reality.
I'm not sure specifically as there could be many things named 'longlegs' based on a true story. It might be a book, a movie, or a play. Without more context, it's hard to say exactly what it's about.