The mouth of the cervix has been identified (initially by Taoist and Tantric philosophy) as a third 'gateway to orgasm', along with the clitoris and vagina. Science is catching up and beginning to study the cervix in relation to sexual arousal and orgasm.
If you don't know what the cervix is, it's basically the end of the uterus where it connects with the end of the vaginal canal. So the 'mouth' of it can be accessed via penetration of the vagina.
Cervical stimulation or pressure causing sexual pleasure makes a lot of sense from an evolutionary perspective. If the clitoris were females only source of orgasm, we would NEVER want to get penetrated, and therefore would never reproduce.
In her book "Vagina: A New Biography" Naomi Wolf posits that cervical orgasms are distinct from other orgasms in that they can feel more emotional.