Greek Orthodox is a type of religion that many people in Greece and other parts of the world practice. It's kind of like a club or group of people who believe in certain things about God and how to follow him.
The people who practice Greek Orthodox believe in a very special person named Jesus who lived a long time ago. They think he was the son of God and that he died on a cross to save us from our sins. They also believe in something called the Holy Trinity, which means there is one God that exists in three different ways- the Father, the Son (Jesus), and the Holy Spirit.
When people go to a Greek Orthodox church, they might hear someone talk about these things during a special time called a service. They also might see some special symbols or decorations like crosses, candles, and even icons (which are pictures of important religious people).
People who practice Greek Orthodox also have some special holidays that they celebrate, like Christmas and Easter. These holidays are very important to them and they often have special foods and traditions that go along with them.
Overall, being Greek Orthodox means believing in God and Jesus, going to church to learn more about them, and celebrating special holidays to remember all the important things they did.