Progesterone is a female sex hormone. Its produced mainly in the ovaries following ovulation each month. Its a crucial part of the menstrual cycle and maintenance of pregnancy. 
 Progesterone helps to regulate your cycle. But its main job is to get your uterus ready for pregnancy. 

After ovulation, progesterone helps thicken the lining of the uterus in preparation for the fertilized egg. When the fertilized egg runs out, progesterone levels drop and menstruation begins. Once the fertilized egg is implanted in the uterine wall, progesterone helps maintain the  lining of the uterus throughout pregnancy. 

Progesterone is required for breast development and breastfeeding. It complements some of the effects of  another female hormone, estrogen. It also acts with testosterone, a precursor of adrenal hormones. Men produce  small amounts of progesterone to help  sperm development.