A thorax is the middle section of a body. If you’re a human, the thorax is your chest, which includes the heart and lungs. If you’re a wasp, the thorax includes the wings and legs. Also, if you’re a wasp, go away.
If you feel pain in your thorax, it may be coming from sore chest muscles or a bruised rib, or it could be more serious, since your thorax includes your heart. Having a bad cough or difficulty breathing affects your thorax as well. Still, this anatomical term is most likely to be used by a doctor or a scientist such as an entomologist, a person who studies bugs. It's the same word, thorax, in both Latin and Greek, meaning "breastplate or chest."