Heartburn (also known as acid reflux) is an uncomfortable burning sensation in your chest. Despite its name, heartburn doesn’t actually affect your heart - it begins in your esophagus.
When you eat, food travels down your esophagus and into your stomach. The muscular valve that separates your stomach and esophagus is known as the lower esophageal sphincter (LES).
The LES has two functions: allow food from the esophagus into the stomach, and keep stomach acids from coming back up into the esophagus. Sometimes the LES allows stomach acids to reflux into the esophagus, resulting in a burning sensation.