In the case of spirometry, after taking the deepest possible breath, there are two values used most often:
- FVC (forced vital capacity) is the total volume of air that the person can exhale. There will still be some air in the lungs even after exhaling to the max possible.
- FEV1 (Forced Expiratory Volume in 1 second) is how much air a person can exhale in 1 second when trying to exhale as fast as they can. As you can see below, since the normal ratio of FEV1/FVC is 0.8, the majority of air can be expelled within one second normally.
Obstructive lung pathology, for example what happens in asthma and COPD, reduces the ability to rapidly expel air due to "air trapping" and therefore the FEV1 drops.
Restrictive patterns like fibrosis do not have that kind of air trapping, so the FEV1/FVC ratio is normal or even higher than normal - but FEV1 is lower because the overall lung volume is lower due to the restrictive pathology.
(Of course this is a simplification; there are mixed patterns and fine details that are out of scope of this question.)
Your actual question about why exactly 1 second as opposed to 0.5 or 1.5 is most likely not answerable beyond the above. Often during research on topics, values are chosen because they are simple to use, and then stick. There are a number of odd values in physiology that were chosen by someone who researched it and now we all memorize it.