The perineum is the piece of skin in between the scrotal base or vulva and the anus.
For girls the perineum extends to the vaginal opening and for guys to the base of the testicles.
It’s very sensitive to stimulation because the area is rich in nerve endings.
When you make love you can include the perineum by touching, stroking or licking it. This feels even better if you make your fingers wet with saliva or a (water based) lubricant.
Some men like the perineum and prostate to be stimulated at the same time.