The vagina is the passageway to the womb.
It is the part of the girl’s genitals that’s on the inside. When you open the inner lips you can see the entrance in between the clitoris and the anus.
Some use the word ‘vagina’ for everything between a girl’s legs. But actually it is just one part of it: a muscular tube that extends from the cervix to the vaginal opening between the inner lips. Vulva (or pussy) is the proper name for the whole (but of course you can name it however you like).
The vagina is not a hole as some assume. The walls of the vagina lie flat against each other and only give way when something enters the vagina, like a finger, penis or dildo.

What does the entrance look like?
How does the vagina feel like?
The vagina & excitement

My vagina stays dry when making love. What’s wrong with me?