It measures the forced vital capacity (FVC). It is expressed as volume (in ml) and is considered normal when it is greater than 80% of its theoretical value.
A CVF between 70 and 65 are the predictable values, they mean a mild disorder, between 64 and 50% moderate, and between 49 and 35 severe. Below 35 very serious.
You are instructed to take a maximum inspiration, and then blow all the air out of your lungs as quickly as possible through a mouthpiece of the spirometer. The patient should wear nose clips and keep the lips tightly closed around the mouthpiece.