The frenulum is a bandlike ligament on the underside of the glans, comparable to the joining ridge under the tongue.
·        The frenulum joins the glans to the foreskin. It stops the foreskin from retracting too far and helps it return to its usual forward position over the glans.
·        A healthy frenulum allows the foreskin to retract freely.
·        It’s the most sensitive part of the penis. With guys that are circumcised only a remnant remains. But this remnant is often still very sensitive and worth your attention when you give a guy a hand or blow job.

Short frenulum: the danger of 'snapping your banjo string'
Torn frenulum