Toward the end of puberty, girls start to release eggs every month which is called the mensural cycle.
Around 1 or two times in a month, during ovulation which is the release of eggs, an ovary takes a tiny eggs and puts/sends it into one of the fallopian tubes where the fertilization occurs.
The egg tends to dry up fairly quickly which usually happens in around 14 days unless of course it's fertilized by sperm.
When the egg leaves the body, this specific process is menstruation (also known as her "period") and is completely normal for girls to have.
Blood and tissue from the uterus join together for the mensural flow which can be a different number of days depending on who you are, but it usually last 3-6 days.
