In the female reproductive system, what is the hymen?

Hymen is a membrane covering the outermost part of the vagina with some tiny holes allowing the monthly menstruating blood to come out.
Types of hymen:
It has no physiological roles. However it is important for medicolegal purposes:

Imperforate hymen : no holes to allow menstruating blood to come out. So the girl is at the age of puberty but without menstruation (which is false – she is menstruating but the blood do not get out of her uterus, accumulation over time causes her abdomen to swell). She might be falsely accused of being pregnant.

Parous introitus means the female have had a sexual intercourse and has delivered a baby. This is important in case of rape especially that of a virgin. Another example is when a woman get married then her husband found out she is not virgin anymore (yeah, she lied about being a virgin).