To understand gastroesophageal reflux disease or GERD, it is first necessary to understand what causes heartburn. Most people will experience heartburn if the lining of the esophagus comes in contact with too much stomach juice for too long a period of time. This stomach juice consists of acid, digestive enzymes, and other injurious materials. The prolonged contact of acidic stomach juice with the esophageal lining injures the esophagus and produces a burning discomfort.

Normally, a muscular valve at the lower end of the esophagus called the lower esophageal sphincter or "LES"-keeps the acid in the stomach and out of the esophagus. In gastroesophageal reflux disease or GERD, the LES relaxes too frequently which allows stomach acid to reflux, or flow backward into the esophagus.