Catholic hamartiology is a big word that describes how Christians who follow the Catholic faith think about sin.
When you do something wrong, it's called a sin. Like when you hit your sibling or take a cookie without asking. Catholics believe that sins are things that drive them away from God and make them not close with Him anymore.
They also believe that there are two types of sins – venial and mortal. Venial sins are like little mistakes that don't separate you from God too much, whereas mortal sins are big, bad mistakes that cause a lot of separation between you and God. For example, stealing a piece of candy might be a venial sin, but stealing a whole bag of candy might be a mortal sin.
But don't worry, Catholics believe that there's a way to make things better after you've sinned. This is called confession, where you tell a priest what you did wrong and ask for forgiveness. The priest then gives you a penance, which is kind of like a little "homework" to do that will help you make up for your sins.
Overall, Catholic hamartiology is just a way to help people understand how to deal with their mistakes and how to make things right again.