I have not had this test, but my husband has. The simplest explanation is that it tests for bone remodeling activity. This gives the doctor a reasonable piece of information to decide if Zometa is right for you. This article from Australia gives a more complex explanation.
https://www.nps.org.au/assets/160b500bb0e8e8b4-42e762c36a33-b4361b6f9bd167a83bc79916dfa0bc09ad7c80ad8347b72b1f36e8d54ae2.pdf#:~:text=Markers%20of%20bone%20turnover%20are%20proteins%20originating%20from,while%20larger%20fragments%20may%20be%20detected%20in%20blood.