The FEV1 is the Forced Expiratory Volume in one second starting from a maximal inspiration. The FEV1 is probably the most important item in spirometry.
If you have determined that expiratory airflow is present, then the FEV1 helps you to assess its severity. The FEV1 can be followed to assess how the patient with obstruction is doing or is responding to treatment. Reversability of obstructive lung disease is probably best assessed by following the FEV1.
The accuracy of the FEV1 is dependent on patient effort. It must begin from a complete inspiration - Total Lung Capacity (TLC). The exhalation "blast" must be as forceful as possible. The first second of the exhalation must be without airflow stoppages (as would occur with coughing, glottic or vocal cord closures). The accuracy of the FEV1 must be confirmed by demonstrating its reproducibility (this is why several efforts are done).