The frenulum is the area just above the urethra or the area just below the clitoral head.
The frenulum is a part of the vulva.

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