The stomach is a muscular bag-like organ that digests food with strong acids contained in gastric juices produced by glands in the stomach lining. To protect itself from being digested along with the food, the stomach lining is coated with a thick layer of mucous produced by mucous cells. This mildly alkaline mucous secretion protects the stomach against the acidic digestive juices.
If the stomach lining is damaged, the stomach sheds off the cells which are quickly replaced with new ones. In severe cases where the protective mucous secretions are inadequate, the stomach lining gets damaged resulting in a painful stomach ulcer.