The male prepuce, or foreskin, is a highly mobile and extraordinarily sensitive double fold of tissue that is the end of the penis. During an erection, it rolls back and inside-out, unfolding until it covers much of the penile shaft. The rest of the time, its specialized mechanisms cause it to spring back into place over the delicate mucus membrane of the glans (head), where it serves a protective function, much like your eyelids.