What does your Progesterone Level mean?

I just got my cd21 progesterone test back today. It was 15.8.. well my doctor told me that i did in fact ovulate. This month I was on 250mg of Clomid and 1500mg of Metformin. I have heard some women on here talk about how doctors say anything over 10 is good and if you're on meds they want it to be over 15. Well mine is right at 15. Is 15.8 on the low side? What does your progesterone # mean?