It is likely you have had this test if you have been diagnosed with COPD, or perhaps your Doctor suspects COPD and has asked you to go for one, it can be very frightening if you don't know what is involved, try not to worry about going for the test.
Basically Spirometry is used to literally measure your breath and involves blowing into a mouthpiece connected to a machine, you'll be blowing several times while the machine takes readings, these readings tell the Doctor how well your lungs are functioning the Spirometry may reveal conditions other than COPD and can be used to help monitor conditions such as asthma. Please feel free to add more to this thread if you know more about this test as my understanding of it is very basic.