"40 under 40" is a list of 40 people who are under the age of 40 and have done really cool or important things in their jobs or in their community. It's like when you make a list of your favorite toys or movies, but instead it's about people who are doing really great things in the world. The people who make the list are usually really smart and work really hard to make the world a better place. They get recognized for their hard work and become an inspiration for other people to do great things too.