MegaGames are a little like an all day board game and a little like Model UN's irresponsible older brother.
A Megagame is a multi-player game, in which, usually the participants are organised into teams, and those teams into an hierarchy of teams. MegaGames are challenging, interesting, and fun. Teams of players take on specific roles - for example as a military headquarters, or a national government, and they interact with other teams representing their opponents or allies.
Honestly, every game is different. Some of them are more like huge wargames and some of them are primarily about diplomacy and negotiation. Mostly, though, they're about communication and information and how sometimes those things are really hard.
