About turkeys... What is the thing called that hangs down from a turkey's beak, and what is its function?

Snood - the flap of skin that hangs over the turkey's beak. Turns bright red when the turkey is upset or during courtship.

It is called a wattle.

"A wattle is a fleshy caruncle hanging from various parts of the head or neck in several groups of birds, mammals and other animals. A caruncle is defined as 'A small, fleshy excrescence that is a normal part of an animal's anatomy'. Within this definition, caruncles in birds include wattles, dewlaps, snoods and earlobes."

"In birds, wattles are often an ornament for courting potential mates."

Besides courting females, males may also use their wattles to intimidate younger, sexually immature males with less well developed wattles. Wattles probably function in thermoregulation as well. In species in which wattles are present in males, the males are also larger in size and the females have fewer caruncles on their heads. Since large bodies lose heat more slowly to the environment, the additional fleshy surfaces may help a male get rid of excess body heat, especially if they are well supplied by blood vessels. It sort of functions more or less like the large ears of the African elephant, which help the elephant lose excess body heat.