Campaign is a pack that contains many maps that share everything between them. It is only single player and can be created using World Editor > Campaign Editor. To create a campaign you must first import a new map from World Editor to campaign editor and then you can start editing that map. Campaigns are common for being story-progressing and lore supported, they are widely used as some kind of a story-lore progress. For more information you should read a tutorial about Campaigns and Campaign Editor. If you understand and have knowledge to create a single map, then it will be no problem for you to create a campaign, it is mostly decided on what kind of genre of map/ campaign you will make. If it is Melee/ RTS then it is gonna be easier, if it is RPG it is gonna be harder.