The ridged band is the really wrinkly part of skin on an uncircumcised penis, where the outer and inner foreskin meet. It contains a really high density of meissners corpuscles (extra sensitive nerve endings).
Most/all of the ridged band is removed with circumcision.
Anatomist Dr. John Taylor reckons that the main function of the ridged band is to trigger a sexual reflexes, such as the reflex contraction of muscles responsible for ejaculation.
The bulbocavernosus reflex has been found delayed or absent in over 70% of circumcised men, which, if John is right, could be due to the removal of that meissners corpuscles filled ridged band.
Note: Avoid googling "magnified view of a ridged band" if you can.