What is a hiatal hernia, and where is the hiatus in your body?

Diaphragm. A hiatal hernia occurs at the spot where the esophagus goes through the diaphragm, a thin muscle that separates the abdomen and chest cavities. When this aperture loosens it allows the top part of the stomach to slide up a little into the chest. It can then result in reflux and heartburn.
Upper abdomen. Hiatal hernia is a bulge of the upper stomach into the chest region. It occurs through the hiatus in the diaphragm muscle, through the passage space for the esophagus and aorta from the chest into the abdomen. Its symptoms are likely to be heartburn and acid reflux into the esophagus.