Hiatus hernia is a structural finding in some people and in some cases it contributes for occurrence of acid reflux.
Most of the patients with hiatus hernia are asymptomatic, without acid reflux (the liquid content of the stomach regurgitates, backs up or refluxes into the esophagus).
Also, it is not necessary to have a hiatal hernia in order to have acid reflux.
Normally, the stomach is completely below your diaphragm - the muscular sheet that separates the abdomen from the chest cavity where your lungs sit.
The weakest part of the diaphragm is the opening through which your oesophagus (the pipe that goes from your mouth to your stomach) passes. A hiatus hernia forms when part of the stomach slides through this opening (hiatus) and into your chest cavity.