Also known as acid reflux, or GERD in some cases, heartburn is a pesky symptom – rather than a condition in and of itself. Heartburn is what happens when stomach acids and other stomach contents reflux or backflow up into the esophagus. Usually, the lower esophageal sphincter (LES) stays closed tightly enough to prevent this from happening. For some people, however, the LES loosens and allows refluxing to happen.