Okay kiddo, you know how different religions have different leaders, right? Like the Pope for Catholics, the Dalai Lama for Buddhists, and so on. Well, one of the leaders for Orthodox Christians is called the Ecumenical Patriarch of Constantinople.
Now some history - a long time ago, there was a big empire called the Byzantine Empire. It was ruled from a city called Constantinople (which is now called Istanbul), and the people there were mostly Orthodox Christians. The person in charge of the church there was called the Patriarch of Constantinople.
Eventually, the Patriarch of Constantinople became really important and started to be seen as the leader of all Orthodox Christians, not just the ones in the Byzantine Empire. So they added the word "Ecumenical" to his title, which means "universal" or "worldwide".
Today, the Ecumenical Patriarch of Constantinople is still seen as the "first among equals" of all the leaders of the Orthodox Church. He acts like a kind of ambassador, going between different groups of Orthodox Christians to help them work together and make important decisions.
So basically, the Ecumenical Patriarch of Constantinople is a really important leader for a big group of Christians, and he helps them all get along and work together.