Okay kiddo, Athleta Christi is a very fancy way of saying "athlete of Christ". It refers to people who follow Jesus and try to live their lives in a way that pleases him, just like athletes try to be really good at their sport.
Just like athletes need to train and practice a lot to get good, Athleta Christi try to learn more about Jesus and how to live like him by reading the Bible and going to church. They also try to be kind to other people and help them when they can, because that's what Jesus taught.
Being an Athleta Christi is kind of like being on a team. Everyone is working together towards the same goal, which is to live a good life and help others. And just like athletes get rewarded for doing well, Athleta Christi believe that they will be rewarded by God for living a good life and following Jesus.