It's derived from the Greek word "polis", which means "city".
Here are some examples:
* Cosmopolitan: A person who is familiar with and at ease in many different cultures.
* Metropolitan: Relating to a large city or its surrounding area.
* New Yorker: An inhabitant of New York City.
* Londoner: An inhabitant of London.
So, "-politan" is used to create words that refer to cities, their residents, or things associated with them.