It's pretty hard to be on this planet and not be recognized as a citizen of somewhere. You can be a citizen of a city, a country, or the world. Citizen can mean "an inhabitant of a particular place."
To be recognized as an official citizen of a town, city, or country, one typically has to meet certain requirements. In return, one gets certain rights, such as the right to vote. In other uses, a citizen may just be someone who lives in a place, such as a "citizen of the world." That's a nice thing to be, but it's not going to get you social security payments when you retire.