Breathing refers to the process of inhaling and exhaling air. Man's respiratory system is inside the chest cavity. This is enveloped by the ribs (skeletons that encloses and protects the lungs from external injury) and diaphragm (a dome-shaped muscular partition that separates the thorax from the abdominal cavity.
Contraction of the diaphragm muscles make it go down, thereby enlarging the chest cavity. Chest muscles are attached to the ribs and the contraction brings the ribs upward and forward thereby enlarging the chest cavity. As a result, air pressure inside the chest cavity is less than that outside the body, so the air outside rushes into the lungs. This is inhaling or breathing in.
When the rib muscles and diaphragm muscles relax, the chest cavity becomes smaller. As a result, air pressure inside the chest cavity is greater than that outside the body, so the air inside rushes out. This is exhaling or breathing out. Breathing is alternate inhaling and exhaling movements.