Menarche is a big word that means when a girl has her first period. You know how sometimes your mommy or big sister gets a stain on her underwear and has to use a special pad that sticks in her underwear? That's because she is having her period.
When a girl starts to grow up and become a woman, her body begins to change in many ways. Some of these changes happen inside her body, like her ovaries starting to release eggs. These eggs are very tiny, but when they join with a special liquid inside the girl's body, a baby can start to grow inside her when she is grown up.
When a girl is around 12 or 13 years old (sometimes younger or older), her body is ready to have her first period. This means that her ovaries will send an egg down a tube called the uterus. If the egg doesn't join with any special liquid that can help it grow into a baby, then the uterus will start to shed its lining (kind of like sweeping up some dust and dirt). This shedding is what we know as the period.
Having your period is a normal part of becoming a grown-up girl, and it is nothing to be scared or ashamed of. Some people might feel weird or embarrassed about it, but it's really just a natural part of being a woman. So if you ever have any questions or concerns about your period, don't be afraid to talk to a trusted adult, like your mom or a teacher. That's what they're there for!