Jean-Christophe Yoccoz was a very smart man who knew a lot about math. He was very good at understanding how things worked and figuring out new things. He studied a part of math called dynamical systems, which is all about how things change over time.
Imagine you have a toy car that you wind up and let go. It moves forward and eventually stops. This is an example of a dynamical system. It changes over time because the car moves and then stops moving. Jean-Christophe Yoccoz studied more complicated dynamical systems, like how the weather changes or how the planets move around the sun.
He figured out a lot of things that other people didn't know about these systems. He was so good at it that he won a really big prize called the Fields Medal. The Fields Medal is like the Nobel Prize for math. It is given to really smart people who make big discoveries that help us understand the world better.
Jean-Christophe Yoccoz was a superstar in the math world and his discoveries continue to help people today.