it has no purpose once you are born. It's a remnant from when the vaginal opening was completely covered to protect the reproductive parts when you were in the womb.
your hymen is a piece of skin that partly covers the entrance to the vagina.
so its inside the vulva, (outer lips) and just inside the inner lips, (vagina)
if you want to see what a vagina looks...
It's the membrane across the vagina in a virgin, usually ruptured when she first has sexual intercourse. It can be ruptured by other means of course. Suggest you consult a medical text book or...