The fourchette is a part of the vulva… and this is the area where the labia minora (inner lips) meet beneath the vaginal opening or entrance and just above the perineum.
This is basically the frenulum in reverse.
See the images below.

The Fourchette And The Female Sexual Pleasure
The fourchette is an erogenous zone and plays an important role in female sexual arousal.
It is rich in sensitive nerve fibers and is extremely and pleasurably sensitive to erotic touch and stimulation during cunnilingus (oral sex).
Particularly, a warm wet tongue on the fourchette feels amazing to women and produces extreme pleasure for women and have them dripping wet in no time.
So there you have it…
…the fourchette.