Pulmonary function tests include tests that measure the efficiency with which lungs function. There occur restrictive or obstructive problems when one inhales or exhales air.

Former relates to airflow problem when the lung tissue or chest muscles cannot expand enough whereas, latter results when air has trouble flowing out of the lungs due to airway resistance. 

PFT measurements tell a lot about lung volume, capacity, rates of flow, gas exchange, and many other parameters.
Test results help evaluate how well lungs perform while they inhale and exhale the air and give oxygen to the rest of the body.