Okay kiddo, so some people believe in something called Scientology and they use a word called thetan. A thetan is like a little being inside you that is really you. Imagine if you had a tiny version of yourself inside your body that is in charge of your thoughts and actions.
Scientologists believe that thetans have been around for a really long time, even before we were born, and that they will keep living after our bodies die. They think that thetans have gone through many lifetimes, or reincarnations, and have experienced all sorts of things.
People who follow Scientology try to learn more about their thetan and make it stronger by doing special exercises and following specific rules. They think that if they can make their thetan stronger, they will be happier and more successful in life.
So, in short, a thetan is like a little you inside your body and some people believe that making it stronger will lead to a better life.