The human stomach stores, breaks down and sanitizes ingested food prior to its entering the intestines for digestion and absorption. It allows humans to eat far more at once than they could actually process through their intestines. It is also very important for certain types of digestion, particularly the digestion of proteins that stomach acids and enzymes affect strongly.
The stomach is a very stretchy organ able to expand up to a gallon in capacity in some people, although the stomach works much better and is more comfortable at half that capacity. The stomach typically stores digested food for one or two hours before beginning to move it into the small intestine. During this time, it releases a substance called gastric juice, which is a mixture of mucus, hydrochloric acid and enzymes. This substance kills many of the bacteria in the food and helps break down various components in the food.
Of course, the stomach itself is made of protein substances, so if unprotected, the gastric juice would break down the stomach as well. To compensate, the stomach generates a thick coating of mucus with enzyme inhibitors and bicarbonate to break down stomach acid. Once the food is released from the stomach, it first travels to the duodenum where it is mixed thoroughly with bicarbonate to neutralize any remaining stomach acid.