The diaphragm is a thin muscular sheet that forms the floor of the chest cavity. The up and down movement of the diaphragm cause change in the size of the chest cavity that is very important for breathing. If it shows no movement the whole breathing mechanism get stopped.
During inhalation, the diaphragm moves down causing the ribs to move upwards and outwards, thereby increasing the space in the chest cavity. This leads to the movement of air inside the lungs.
During exhalation, the diaphragm moves to its former position and the ribs move downwards and inwards, thereby reducing the size of the chest cavity. This leads to the movement of air out of the lungs.