Progesterone Check?

I am on my third month of Clomid, taking 150 mg days 5-9. My gyno's nurse told me that on day 21 of my cycle (which is the 28th) I would need to come in for lab work to see if my body is responding to the Clomid or not. She said they were going to do a Progresterone check? I've researched and found out it's a hormone that has a million essential jobs to concieving a child. What I don't understand is are they checking to see if I have too much or little of the hormone? Or if I have it at all? What exactly are they checking for?