While playing Campaign mode, there are parts of each mission called objectives that inform you about what needs to happen next to advance the story. When a new objective comes up, the game will momentarily pause and the new objective will be displayed on screen. Also, the objectives of the entire level are cataloged on the map display screen. These objectives may include: killing so many enemies, protecting a certain point, destroying an object, going to a location, or transporting an object to a certain place. Once you complete all the objectives in a level, that level is beaten.
All the objectives of each campaign mode level can be seen in the synopsis of each level.