Stomach acid breaks down food so that it is more easily absorbed by the body, according to MedicineNet. Muscles in the wall of the stomach churn the food, stomach acid and enzymes together to create a thick liquid, known as chyme, that moves into the small intestine for further digestion.
Wikipedia states that stomach acid, also known as gastric acid, breaks down proteins in food. First the stomach acid causes the structure of the protein to unfold to make the bonds between the protein's amino acids more accessible. Then the gastric acid activates enzymes that destroy the bonds. Stomach acid consists of hydrochloric acid, sodium chloride and potassium chloride, which are produced by the stomach lining. The hydrochloric acid also helps to destroy some harmful microorganisms in the stomach to help prevent disease. When the chyme moves along to the small intestine, it mixes with sodium bicarbonate, which neutralizes some of the acid to prevent injury to the rest of the digestive system.
MedicineNet describes one of the other protections that the body has to prevent harm from stomach acid. At the juncture between the esophagus and the stomach, there is a one-way valve that keeps acid from flowing back into the esophagus. Wikipedia notes that the stomach itself secretes a thick mucus, which coats the stomach lining and keeps the acid from harming it.