Forced Expiratory Volume in 1 second. This is the amount of air you can blow out in one second. This is the number, I am told, that is most relevant. I understand this is usually quite low in CF patients, even if the other two are high. I guess this is where the oxygen gets to the blood.
Barometric pressure can play a role too - new machines correct for it themselves.
By comparing present tests to previous - that can help. It is helpful if you can have a similar routine prior to having tests - that makes the trending that much more accurate. Everyone's lung function has variations over the course of the day - so time of day is important too. Ask your friendly PFT person if you want to know more. Studies have been done with thousands of people to develop these predicted values.