So imagine you have two friends - one friend is really good at making money and has lots of toys, while the other friend doesn't have as many toys and struggles to make money.
Now imagine that the first friend, who has lots of toys, decides to take some of the other friend's toys without asking. This makes it harder for the second friend to play and have fun.
Well, a long time ago in history, European countries (the first friend) decided to take over many countries in Africa (the second friend). They did this by force, by trickery, and by making unfair deals with the rulers of the African countries.
Once they had taken over these countries, the Europeans made it difficult for the African people to make money and grow their own countries. They took all the resources - the gold, the diamonds, the rubber - back to Europe where they could make even more money.
This meant that the African countries couldn't develop their own industries or grow their own economies very well. They were left behind while Europe became rich and powerful.
It's like if the first friend kept taking more and more of the second friend's toys, that friend wouldn't have anything left to play with and wouldn't be able to have as much fun.
So Europe underdeveloped Africa by taking their resources and not letting them develop their own industries and economies. This made it very hard for African countries to grow and become as rich and powerful as European countries.