Milliarium aureum was a very important structure in Ancient Rome that was kind of like a big signpost. It was made of shiny gold and marked the center of the Roman Empire.
Imagine you're playing a game of "Follow the Leader" with your friends. When you get to a really important part of the game, you might make a special mark or a big sign to show everyone that they've made it to an important point. Milliarium aureum was kinda like that special mark for Ancient Rome.
When people arrived at the milliarium aureum, they knew they were at the very center of the Roman Empire. It was like a big, shiny reminder that Rome was the most powerful city in the world at that time.
So basically, milliarium aureum was a golden marker in Rome that showed people where the center of the Roman Empire was.