Hiatus just means gap or hole (in Greek). The gullet (oesophagus) is the food passage from your mouth to your stomach. To get there it has to pass through a hole in a flat sheet of muscle that separates the chest (containing your lungs and heart) from your abdomen (where your guts live).
So the gullet passes through a hole or hiatus. Sometimes this hole enlarges and the top of the stomach passes upwards into the chest – and this is called a hiatus hernia. The tricky part about fixing a hiatus hernia is that you can’t just close off the hole, because you always have to leave a small gap for the gullet to pass through. So the ‘repair’ has to narrow the opening just the right amount – not too little so the repair fails, and not to much so that the passage of the food is prevented.
Nowadays the repair is usually done laparoscopically. The classical operation was a ‘wrap around’ procedure, and more and more surgeons are using mesh. It is a really technical thing, and difficult to get right with good results.
First you need to decide if you really want or need an operation. Often the symptoms can be controlled with some weight loss, careful diet and medicines.
If you are thinking of having the hiatus hernia repaired surgically, make sure you find an experienced or expert surgeon, and try to guage how confident he / she is about getting a good result and a long term improvement for you.