I'm glad you asked. First, skip around this video to a few random points to get an idea:
Did I grab your attention now? Great. Here's the gist:
MegaGames are a very specific and rare genera of game that is played as an event in a physical location with a large number of players all at the same time. Most common at conventions, but very occasionally run elsewhere on their own, MegaGames offer a very unique roleplaying and gaming experience that is entirely unlike any other form of media, with the possible exception of LARPing, which is a close cousin of MegaGames.
I was getting to that.
The shortest answer is that it's like a board game for 25-150 players at once. Most MegaGames average around 50-70 players and range between 2-10 hours to play. Unlike some LARPs, they rarely last more than one day. Players usually assume the roles of characters and are given objectives that they will try to satisfy over the course of the game. If you are familiar with games like Werewolf, Mafia, Among Us, or Blood on the Clocktower, then a lot of those elements are present in most MegaGames, often with political themes.
MegaGames as a format generally got their start as British Wargames and as such there is a strong contingent of the MegaGame community that is about history reenactment. However in the United States, games like Watch the Skies (Think Model UN meets X-Com), Den of Wolves, (Space Opera) and others often focus on sci-fi, political, and apocalypse scenarios.
Visit the MegaGame Coalition's Discord server, or look at their list of upcoming worldwide MegaGames. If you would like to play one anywhere near the state of California, then do come to one of ours! We often run them at our convention: Game Diego.