It means the inflow [inspiration] and out flow [expiration] of air between atmosphere and the alveoli of the lungs. During inspiration lungs are expanded and during expiration lungs are contracted. There are mainly two processes by which the lungs are expanded and contracted.
1) The downward and upward movement of the diaphragm which increases and decreases the diameter of the thoracic cavity.
2) The elevation and depression of the ribs due to contraction and relaxation of intercostal muscles lengthens and shortens the thoracic cavity.
Inspiration: Thoracic cavity expands by the upward and outward movement of the ribs. It is followed by flattening of the diaphragm, brought about by the contraction of the muscles of the diaphragm. With an increase in the volume of the thoracic cavity, the pressure here becomes lower than the outside atmospheric air which forces air into the lungs through the nose and trachea.
Expiration: Expiration is mainly a passive process. Relaxation of muscles of diaphragm and inter costal muscles moves the ribs and thoracic wall inward and downward, which decreases the volume of thoracic cavity and increases pressure in thoracic cavity which causes air to be expelled out.