Normally, food passes from your mouth, to your throat, through your esophagus, past the ring-like muscle called the lower esophageal sphincter, or LES, and into your stomach. Food moves through your esophagus because of muscular contractions known as peristalsis. The LES then relaxes or opens to allow food into your stomach. Then, it quickly closes so that neither food nor the acidic contents of your stomach can pass back up into your esophagus.
With GERD, the LES either doesn't close properly or relaxes when it shouldn't, so the acidic contents of your stomach flow back up into your esophagus. This is called reflux. Reflux occurs in everyone from time to time, but if it happens too often, it can damage your esophagus' protective lining.