Gastrooesophageal means stomach and esophagus. Reflux refers to flow back or return. Therefore, Gastroesophageal reflux disease refers to the return of the stomach’s contents back to the esophagus.
At the entrance of the stomach, there is a valve which is a ring of muscle called the lower esophageal sphincter. Normally this sphincter closes as soon as the food passes through it. If it does not close or remains open, acid produced in the stomach move up in the esophagus.