What Is A Serum Progesterone Test?

what is a serum progestorone test? I a 33 years .5.2 feet height and 69kg weight . I am married for four years and last month for first time I consulted a gynocologist and went through a medicine course for five days (folicite) send day after periods. and did a 12th day scan and 13th day a ingection (not sure ends with 5000)was given to I think to strenghthen the egg and after 36 hrs was tol to have sexual intercourse and on 21st day I have to go through the serum progestorone test. pls' explain why these test for and what is this Serum prgn test. thank you.