What does it mean when a girl is ON her period?

What does it mean when a girl is ON her period?-I know what a period is and the menstrual cycle etc. but when people say having s3x on the girls period what does that mean? Like you have s3x the day she is supposed to get it? And I've heard its very messy. But does it still count as having s3x on her period if she doesn't bleed? Explain what it means for a girl to be "ON" her period.

  When a girl is on her period she is having her menstrual cycle and bleeding.
The first 2 days are heavy and the other days lighter until no bleeding. 

Can last up to a week.
Technically a girl on her period who is not on any hormonal birth control should be bleeding.
Some girls that are on birth control might not have any bleeding and their periods and bleeding is not really the same as a girl not on birth control hormones. 

The ones are BC hormones do not have a real bleeding but an induced or artificial bleeding as there is no egg released to be fertilized when a girl is on birth control hormones. 

Every month an egg is released in a fertile females body and travels to the uterus (located in the vagina and where the egg is fertilized by the sperm and where it stays for 9 months while the embryo becomes a baby) if this egg is not fertilized than the uterus sheds it's lining and the female has a period. Then the cycle starts again. 

A female on BC hormones does not release any eggs therefore there really is no shedding of the lining of the uterus so maybe that is why there is no blood.