Your friend is 9 and she told you she got it??!! Ok, besides that, I will tell you what a period is. When a girl's body tells her to get ready to have kids and make kids, her body starts sending hormones to tell her to grow. One of the changes is a period
*Down there*
Inside the girls vagina (or uterus) she starts getting a padding of blood in there, getting ready to hold a fertilized egg. (or baby) This is a monthy occurance. Then, when she doesn't have it fertilized, the blood comes out her vagina. this is a PERIOD. It usually last about 2-7 days. It is a cycle, and it does it over and over every month. (but when the girl first gets her period, it might not be) It only stops doing it when the girl is pregnate or has has stopped getting it. (around 45-55 years old.)