The ridged (frenar) band is part of the foreskin: the elastic tissue at the interface between the inner and outer foreskin. Normally it is hidden from view, just inside the tip of the foreskin.
It helps the tip of the foreskin to contract, allowing it to remain positioned over the glans. When the penis is soft it tightens to narrow the opening of the foreskin. It often “puckers” past the tip of the glans.
Because of its elastic properties it allows the foreskin to be retracted. During erection it forms a band halfway down the shaft, all the way around.
It’s a delicate belt of sexually responsive tissue.

What is the connection between frenulum and ridged band?

If the ridged band is too tight, making it unable to retract the foreskin over the glans, you have to stretch it.