Okay kiddo, imagine you're putting together a big puzzle of the world's economic history. The Maddison Project is like a big book with lots of information that helps us put those puzzle pieces in the right place.
The book tells us how much money different countries made a long, long time ago - even before you were born. It's important because we can learn from the past and use that information to make better decisions in the future.
For example, the book might tell us that a long, long time ago, a certain country was doing really well and making lots of money. We can look at why that was and maybe try to copy some of the things they did to make our own country better.
So the Maddison Project helps us fill in the gaps in our puzzle of the world's economic history, and lets us learn from the past to make a brighter future.