A campaign contains all the chat settings of your chat window, like the color, proactive greeting message, on which pages you place the chat window, etc.
You can install multiple campaigns in case you have differentiating goals with the chat on your website.
Categorizing your campaigns can be important when you want specialized chat operators to chat with specific customers. It is for example important to get in contact with customers that wish to downgrade their subscription. You most likely want your best chat operators to be talking with these customers. On the other hand, support questions are supposed to be handled by your customer service. By installing different campaigns these different types of website visitors can be greeted in the appropriate manner and helped by the right chat operators.