Isamu Mochizuki is a person, just like you and me! He's from Japan, which is a country far away from where we are.
Isamu Mochizuki is special because he's really good at math. Math is like a big puzzle that you have to solve using numbers and symbols. Isamu Mochizuki is really good at solving very hard math puzzles, ones that most people can't solve.
One of the things Isamu Mochizuki is famous for is something called the "abc conjecture". This is a very hard math problem that lots of people have tried to solve, but nobody has been able to do it yet. Isamu Mochizuki thinks he has figured out the solution to this puzzle, but it's really complicated so it's hard for other people to understand.
Even though Isamu Mochizuki is really smart and good at math, he's still just a person like you and me. He likes to spend time with his family and friends, and he enjoys doing other things besides math too. But he's really good at math, and that's why people know his name.