Heartburn is a burning sensation in the upper abdomen
Gastric acid is intended to digest food quickly and effectively, and is therefore a pretty aggressive liquid. The stomach is provided with a thick mucous lining and therefore protected, but the esophagus does not have it and is therefore fragile. Normally, the transition between stomach and esophagus is closed and opens to leave food to the stomach and immediately closed again. When this closure does not work well enough, acid from the stomach can flow to the esophagus, which is also called reflux. This causes the burning sensation. There are medications available to counteract the effect.