An anatomical structure centered on the join between foreskin and glans of the cock on its underside. In particular, there is usually a ridge of tissue joining the interior of the foreskin and the base of the glans; it is, developmentally, an extension of the ridge on the underside of the cock where the two tissues (which would have been the inner lips had the fetus not been set on a male development path at about 8 weeks after conception) would have developed as inner cunt lips. Developmental derangement sometimes results in incomplete fusion of these tissues, resulting in a longitudinal opeing of the urethra.
Radically circumcised cocks (ie, those with completley absent foreskins) do not, properly, have a frenum as there is no join between tissues. However, as the frenum is, in most men, the most highly concentrated source of exciting stimulation, an important aspect of the frenum is the concentration of nerves (especially sensory ones) at the site. Some of these nerves may still exist even in radically circumcized cocks.
The sensitivity of the area varies from man to man and with time. Some find stimulation of the area, as with a tongue in fellatio or cock worship, too intense to endure. And, perhaps expecially just after coming, sensitivity of the area may become similarly intense.