The Fontan operation is a reconstructive open-heart surgery in the treatment path for many heart defects, including HLHS.
This surgery is usually preformed when the child is 1.5-3 years old. This surgery redirects deoxygenated blood directly to the lungs instead of through the heart. Doctors accomplish this by detaching the inferior vena cava from the heart and attaching it to the pulmonary artery. This relieves stress on the heart and improves oxygenation of the blood.
Here is the University of Iowa's explanation of the Fontan.
Here is Children's Hospital of Philedelphia's explanation of the Fontan as well. This one is particularly helpful because it explains the previous steps involved and uses HLHS (the condition John has) as it's example.