The skin under a rooster's neck is called a wattle. Roosters also have a fleshy growth on top of the their head called a comb. Hens have smaller combs and wattles than roosters.
The wattle serve several functions. Chickens do not sweat in the heat; instead, their bodies cool down by circulating blood through the comb and wattle. Roosters use the wattle during their mating dance, known as tidbitting, to attract females. Larger wattles have been associated with better nutrition, the ability to avoid predators and a higher level of testosterone. Hens associate roosters with larger wattles as having more potential to be successful mate.