Each month, the female during her reproductive years, will produce an egg. This happens because of a complex and very well orchestrated sequence of events. In short, the period starts when the blood level of the female hormones Estrogen and Progesterone drops. The pituitary gland then starts producing FSH (Follicle Stimulating Hormone) that makes the follicles (eggs) grow. As the egg is growing, it produces Estrogen hormone that among other things, prepare and thicken the lining of the uterus.
When the egg and the Estrogen reach certain levels, the LH hormone level peaks and ovulation occurs (the egg actually gets released from the ovary). Fertilization then occurs within window of about a day or so. Of note is that sperm cells are alive and active for about 2 days or so after intercourse. So, we can get pregnant if intercourse has occurred a day or two before ovulation. But if intercourse has occurred 2 or more days after ovulation, we are less likely to get a pregnancy that month.
After LH and ovulation, we see a rise of the Progesterone hormone. This continues for about 7-9 days. If no fertilization occurs, and no pregnancy, Estrogen and Progesterone hormones start falling and the following period starts. And the cycle repeats.