The foreskin is the double-layered fold of smooth muscle tissue, blood vessels, neurons, skin, and mucous membrane part of the penis that covers and protects the glans penis and the urinary meatus.
Purpura et al. (2018) describe the foreskin as follows:
Few parts of the human anatomy can compare to the incredibly multifaceted nature of the human foreskin. At times dismissed as “just skin,” the adult foreskin is, in fact, a highly vascularized and densely innervated bilayer tissue, with a surface area of up to 90 cm, and potentially larger. On average, the foreskin accounts for 51% of the total length of the penile shaft skin and serves a multitude of functions. The tissue is highly dynamic and biomechanically functions like a roller bearing; during intercourse, the foreskin “unfolds” and glides as abrasive friction is reduced and lubricating fluids are retained. The sensitive foreskin is considered to be the primary erogenous zone of the male penis and is divided into four subsections: inner mucosa, ridged band, frenulum, and outer foreskin; each section contributes to a vast spectrum of sensory pleasure through the gliding action of the foreskin, which mechanically stretches and stimulates the densely packed corpuscular receptors. Specialized immunological properties should be noted by the presence of Langerhans cells and other lytic materials, which defend against common microbes, and there is robust evidence supporting HIV protection. The glans and inner mucosa are physically protected against external irritation and contaminants while maintaining a healthy, moist surface. The foreskin is also immensely vascularized and acts as a conduit for essential blood vessels within the penis, such as supplying the glans via the frenular artery.