Okay kiddo, so do you know what GDP means? It stands for Gross Domestic Product, which is basically the total value of all the goods and services made in a country.
Now, Angus Maddison was a really smart man who studied the economies of different countries. He looked at their GDP and made a list of the ten countries with the largest GDP when you adjust for what things cost in each country. This is called Purchasing Power Parity or PPP for short.
So, think of it like this: let's say you have ten friends and they each have a different amount of money. If you all went to buy ice cream, some of your friends might be able to buy more because they have more money. But if you adjust for how much things cost where they live, some of your friends might actually be able to buy more ice cream than you thought!
That's what Angus Maddison did - he looked at the GDP of different countries but made sure to adjust for how much things cost in each country. And he found that the ten countries with the largest GDP (PPP) were: China, United States, India, Japan, Germany, Russia, Indonesia, Brazil, United Kingdom, and France.
So basically, PPP is like making sure everyone has the same amount of "money" when looking at the economy, and Angus Maddison made a list of the countries with the largest "money" in this way.