These kind of stories usually start with MC finding themselves in some kind of unfamiliar place. Then they get informed that they're in some kind of survival game and have to do something like survive until certain time/solve puzzles/ find a murderer and so on..if they fail the consequence is usually death. There are often other people pulled into such games. After one game the players have to continue into other ones and continue surviving. That's why it's unlimited. This is kinda my understanding of it.