Unique among other mammals, Humans remain "on heat" all year around - otherwise human society would not be possible! The inherited pattern of hormones a woman produces cause one egg to become fertile every month and this process involves washing away the unwanted eggs when they are not used. That bleeding is called "menstruation" - also 'Eve's Curse.' But don't curse it too much - imagine the chaos if all women came on heat over a fixed fortnight once a year! "Chaos" would be an understatement!!!
Once a month, an ovum is released from the ovaries for the purpose of spawning a child. The uterus creates a protective layer made up of blood and tissue to provide protection for the ovum should it be fertilised, so that it can grow into a foetus, then into a living, breathing baby. If the egg isn't fertilised, then there is no need for the protective layer within the uterus, so it expels itself through the vagina. That's called a period, and that's basic Sex Ed that I learnt in the third grade.