Okay kiddo, so you know how when you go to church, there's usually someone in charge, like a pastor or a priest? Well, in the Orthodox church, there is also someone in charge, called a Patriarch.
Now, the Orthodox church is split up into different branches, kind of like how a tree has different branches. One of these branches is the Turkish Orthodox church.
But sometimes, the church in one country wants to be more independent and have their own leader instead of following the leader of another country. This is what is called autocephalous- it means "self-governing".
So, the autocephalous Turkish Orthodox Patriarchate would be the Turkish branch of the Orthodox church having their own leader and being able to make their own decisions without always having to follow the leader of a different country.