The phrase "main campaign" refers to the main or primary storyline of a game, movie, or book. It is the central or most important part of the overall story that the audience follows. Full definition
According to the report, player actions in main campaign missions will affect the direction in which the story moves. (techradar.com)
There's only the co-op mode to play in which is basically just playing through the same 9 missions that is featured in the game's main campaign mode. (justpushstart.com)
The mode greatly increases the game's content factor after the 10 - 20 hour main campaign. (cogconnected.com)