Gastroesophageal reflux disease or GERD, refers to chronic heartburn that occurs at least twice a week. The pain of GERD results from acid in stomach contents flowing backward in the esophagus - the tube connecting the mouth and stomach. The acid irritates and burns the lining of the esophagus. This backward flow of stomach acid results from leakage of a valve where the esophagus joins the stomach. As GERD progresses, the valve may be scarred by acid exposure, so that leakage increases, and the esophagus is damaged further.