Okay, kiddo, let me tell you a story about an old city called Rome. A really long time ago, Rome was the most important and powerful city in the world. It had great rulers, impressive buildings, and a strong army.
Now, fast forward a few centuries, and Rome is no longer the center of power it once was. A new city called Constantinople has emerged as a powerful empire, but it still looked up to Rome's greatness.
In fact, the people of Constantinople thought their city was so great that they called it the Second Rome. Now, fast forward a few more centuries, and Constantinople fell to the Ottomans. But, the Russians, who were eager to become the most powerful nation in Europe, saw themselves as the rightful heirs to the mantle of Rome.
So, they claimed that Moscow was the Third Rome. They saw themselves as the protectors of the Orthodox Christian faith, just like the first two Romes had been the protectors of Christianity. The Third Rome was a symbol of Russia's power and might.
So, there you have it, kiddo. The Third Rome was a term that the Russians used to show that they were the most important nation in the world, just like Rome had been in the past. They believed that they were carrying on the legacy of the ancient Roman Empire and that they would become the most powerful nation in Europe.