Cardinals are special people in the Catholic Church. They are like superheroes who help the Pope make big decisions and run the Church.
Back in 1929, a Pope named Pius XI wanted to make some changes to how cardinals were picked. He created a special law called the "Apostolic Constitution" which laid out how cardinals would be chosen from different parts of the world.
This new law was kind of like making a game with rules. It made sure that there were always enough cardinals from different countries and backgrounds to make fair decisions for everyone in the Church.
Think of it like playing with your friends, but making sure everyone gets a turn to pick the game and have fun. That's what Pius XI did for the Church by creating the cardinals.