Answer
Menstruation is the normal bleeding coming out from the vagina due to degeneration of the uterine wall which was developed over a cycle of around a month, which is important for the fertilized egg to develop into an individual.
When the egg is not fertilized by the sperm the wall of the uterus degenerates and comes out of the vagina, as they have dense blood supply these blood vessels also degenerates which makes the blood like appearance of the degenerated walls.