You are a citizen of many different things. You belong to a city and a neighborhood, a state or province, and a nation. The word “citizen” is something like a title in that it implies there are certain rights and privileges that are at your command, in addition to a few obligations to maintain the entity.
More important, however, is that citizenship is an identity. While the various actions required by law or custom only come up once in a while, your kinship is a constantly defining force. You might travel around the world and meet someone who shares a citizenship with you, and there is an instant bond.
But what does any of this mean to the ever increasing number of people who call themselves “global citizens”?