What is a hiatus hernia?

A hiatal hernia?: I'm guessing you are referring to a hiatal hernia. You will likely benefit from a diagram here (calling Dr. Google). There's a large muscle called the diaphragm that separates the thorax from the abdomen. Most of the time, the esophagus is above the diaphragm, and it connects to the stomach below the diaphragm. In a hiatal hernia, the stomach "begins" ABOVE the diaphragm. But do find a picture!

Stomach in chest : Have a look at this link https://www.healthnavigator.org.nz/health-a-z/h/hernia-hiatus/ Basically some of the contents of the abdomen (usually the stomach) can be pushed up into the chest through a small defect in the diaphragm. Most of the time it needs no treatment and most people have no symptoms. It is commonly picked up on X-ray or gastroscopy.

Gullet in chest: As dr large said stomach contents in chest doesn’t require treatment if symptoms. Some people use the term more colloquially for what we call gastroesophaheal reflux. It can usually be treated with lifestyle or medication changes