These are games that are modeled after the books/movies, The Hunger Games. They usually are a major feature of a server and are hosted on their own servers separate from other gamemodes the network runs.
The idea is that you start off with nothing facing a central Cornucopia which contains food, weapons and other useful items such as armor pieces. There is an initial rush to the center after a short countdown and players fight for whatever resources there are. Players continue to fight each other until there is only one left standing, The Victor.