Okay kiddo, imagine a group of people who believe in a certain religion and they have a leader called "patriarch". Now, this particular group of people is called "Armenians" and they have their own patriarch whose job is to take care of their religious needs and make important decisions for them.
The patriarch of Armenians in Constantinople is a very special leader because he is responsible for the Armenians who live in Istanbul, Turkey. Constantinople is actually the old name for Istanbul.
So, even though the Armenians are a small group of people in Turkey, they still have a leader who is there for them and helps them with anything that they need. And that leader is called the Armenian Patriarch of Constantinople.