The longer answer
The bead is a small bright bump — sometimes a brass dot, sometimes a glowing fibre-optic pipe — set into the rib at the muzzle end of the barrel. Many target guns also carry a smaller mid-rib bead halfway down. When the gun fits and is mounted correctly, the front bead sits at the top of the rib in your peripheral vision; if there's a mid-bead, the two form a "figure-8" or stack into one another to confirm the eye is centred.
A shotgun is not a rifle. You don't aim at the bead the way you align a rifle's sights; you look at the target, and the bead is a peripheral confirmation that the muzzle is where it should be. Stare at the bead and you'll stop the swing — and stop hitting birds.