Everyones body has a perineum or perineal area. In men, it is the area is between the scrotum and the anus, in women it is the area between the bottom of the vaginal opening and the anus.
During sexual arousal and excitement the area may become more sensitive, and some people find perineum massage highly pleasurable.
For men, massaging the perineum can provide external stimulation of the prostate and even the root of the penis, both of which can be pleasurable.
A common slang term for the perineum is "taint".