The frenum is an anatomical structure centred on the join between foreskin and glans of the penis 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 penis where the two tissues (which would have been the inner lips had the foetus not been set on a male development path at about 8 weeks after conception) would have developed as inner cunt lips (ie, labia minora). Developmental difficulties sometimes result in incomplete fusion of these tissues, resulting in a longitudinal opening of the urethra. In general, the reasons for these difficulties are not known, though the presence of chemicals with oestrogenic effect (natural or artifical) in micro amounts in the environment (and foods) are increasingly suspected.
Radically circumcised penises (ie, those with completely absent foreskins and sometimes extensive scars) do not, properly, have a frenum as there is no join between tissues. However, as the frenum is, in most men, the most intense source of exciting stimulation, an important aspect of the frenum is the concentration of nerves (especially sensory ones) at the site. Some of the sensation is probably due to stretch receptors, which will generally have been rendered moot with the removal of strechable tissue during radical circumcision. Some of these nerves may still exist, and retain some function, even in radically circumcised penises.
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.