Civic nationalism is, in a way, a positive vision of what nationalism should be. It is one that is based on citizenship and legal rights, not on biology.
One form of nationalism is based on race. In this form of nationalism, people should feel tied to one another because they are of the same "race" or ethnicity. In this sort of nationalism, no one can choose to belong or not to belong to some nation. They are in that nation because of their birth.
By contrast, civic nationalism is based on choices we make. If I go to Canada and I want to embrace the values and attitudes of Canada, then I can be a Canadian. I do not have to be of any particular ethnic group to be included as a Canadian. All I have to do is to buy into the attitudes that make a person Canadian and then make the choice to become a Canadian citizen.
These are very different views of what should tie us together. In ethnic nationalism, we are tied together by our "blood." In civic nationalism, we are tied together by our values, our attitudes and our choices.