What is the significance of change in your pulmonary function test after using bronchodilator?

See below. Bronchodilators are given during a PFT to see if you respond. If you do, that means giving you medications like albuterol would help you breathe better and would be a good option for treatment.
Probably asthma. An improvement in pulmonary function as measured by spirometry after using a bronchodilator is indicative of the presence of reversible bronchospasm. Bronchospasm is caused by contraction of smooth muscle cells in the walls of larger breathing tubes. This is a common finding in asthma. So, in short, it helps cement a diagnosis of asthma.