Hello Lilian, the concepts of decision trees and entropy are actually machine learning concepts. The project enabled me to understand how I can apply machine learning on data analysis projects to draw more insights so if you really want to know these concepts, it's best to start with:
1. What is Machine Learning
2. Types of Machine Learning
3. Types of Supervised Machine Learning.
4. Decision Trees

Google can help a great deal. I advise you follow these steps so that you won't get lost if you start with decision trees.

You can also browse on several techniques in Churn Analysis too.

I apologize for not dropping any link. I can't really pinpoint the particular link that helped because I used a combination of several links to understand it.