What does that mean? Well, way back in the day, when you went to conventions to play games, all of them were what is now called “classic” format. You showed up and either they had pregen characters ready for you, or there was a quick chargen as part of the session.
Then, they came up with the idea that for large, recurring stuff it would be cool if you could play the same character, ideally your character, from game to game. Thus was born the Living City, set in the Forgotten Realms. You could generate your own character according to slightly-modified 2e rules, and in each session you got XP and treasure you could take to the next game. This required a little complexity and was only suitable for larger efforts, because you had to split players into level ranges and whatnot. (Characters couldn’t “port” between home games and the Living campaign for strict fairness – you can only advance them within the campaign.) And thus was born the “campaign” format of organized play.