The longer answer
The rib is a sighting plane, not a sight. A shotgun isn't aimed the way a rifle is — there's no rear notch, no precise picture to settle. What the rib gives you is a long, level reference that lies between your eye and the bird. When the gun is mounted correctly, the cheek sits on the comb, the eye drops into the rib's line, and the front bead floats at the far end like the dot at the end of a sentence.
Most field and clay-sport guns wear a flat rib — a thin, ventilated strip running flush along the top of the barrel. Dedicated target guns often carry a stepped or raised rib, which lifts the sighting plane slightly so the shooter sees the bird above the barrel rather than behind it. Both work; the choice is about how high you want the gun to shoot relative to where you look.