What is a cornucopia?

To get a solid visual of a cornucopia, consider a traditional Thanksgiving table. Typically on this table, a cornucopia is represented: a long, horn-shaped object that is closed and narrow at one end and open and wider at the opposite end. The cornucopia is usually filled with items that are found in an autumn harvest: pumpkins, squash, and corn, and sometimes with other fruits and even flowers. A cornucopia often symbolically represents an abundance, an overflowing of what is needed to survive.
In The Hunger Games, the cornucopia is the center of the tributes's starting point. Within this cornucopia is an abundance of what is needed to survive, but because it's the Hunger Games, this contrasts sharply with the typical Thanksgiving table. In this cornucopia are backpacks with supplies, swords, food, bows with arrows, and protection from the elements. And, of course, they also fight the final bloodbath at the cornucopia, bringing devastation to a symbol typically associated with abundance.