Alright, kiddo, imagine that your tummy has a really strong gate between it and your throat, so that the food you eat stays in your tummy instead of coming back up. But sometimes, that gate gets weak and doesn't stay closed all the way, so the food and acid from your tummy goes up into your throat and maybe even your mouth.
When that happens, your throat and sometimes your chest feel really hot and kind of burny, like when you accidentally touch the stove. That's why we call it heartburn! It doesn't have anything to do with your heart, it's just where you feel it.
Sometimes, eating certain foods or drinking certain drinks can make that gate weak and cause heartburn. Some people might need some special medicine to help their tummy make less acid so they don't feel it as much. But mostly, it just means avoiding foods that make your tummy upset and eating slowly and calmly so your tummy can digest food better.