The Ecumenical Patriarchate of Constantinople is like the boss of all the Orthodox churches in the world. Just like how your school principal is in charge of all the teachers and students in your school.
But instead of a school, it's a group of churches that follow the same religion called Orthodoxy. And these churches are located all around the world!
The Ecumenical Patriarch is kind of like the leader of this huge group of churches. They work hard to make sure everyone is following the same rules and staying true to the beliefs of the Orthodox Church, just like how your teacher makes sure everyone in class follows the same rules and learns the same things.
And just like how you might have a favorite teacher or subject, the Orthodox Churches also have their favorites within the church. The Ecumenical Patriarch is the most important of all the bishops in the Orthodox Church and is based in Istanbul (which used to be called Constantinople a long time ago).
The Ecumenical Patriarch has been around for almost 2,000 years and has played a big part in shaping the history of Christianity. Even today, they continue to help guide and support the Orthodox churches around the world.