Okay kiddo, the IAU Trail World Championships is like a big running race where people from all around the world get together to see who can run the farthest and the fastest on a really tough and hilly course. The runners have to use their feet to climb up steep hills, jump over rocks and roots on the ground, and run down slopes without tripping or falling.
The race is organized by the International Association of Ultrarunners (IAU) and it happens once a year in a different location every time. It's kind of like a big Olympics for runners who love to take on challenging trails.
The race is made up of different categories for men and women, and the runners have to qualify to compete by running in other tough races beforehand. The people who win the race get to show off their medals and feel really proud of what they accomplished.
It takes a lot of hard work and dedication to be able to run in the IAU Trail World Championships, but the feeling of accomplishing something like that can be really special.