What is heartburn?

When the stomach acid backs into esophagus giving a burning sensation is called heartburn

 
There is a muscle called Lower Esophageal Sphincter (LES). LES functions as a control valve between the stomach and the esophagus. When will swallow food, it goes from the esophagus through this valve which (LES relaxes) opens up allowing the food to enter the stomach, and then valve closes back up.
Stomach has and generates acids to digest the food. The stomach is immune to these acids. If pressure is exerted on the stomach and the valve relaxes, some of the food along with these acids reflux out of the stomach back into the esophagus. While the stomach is immune to the acid, esophagus is not. It results in a chemical burn giving a burning sensation. This is called the heartburn.
The LES can get damaged from this acid. However, if the heartburn is properly controlled, the LES recovers to normal within seven weeks.