Alright kiddo, do you know what a country is? Okay, so back in the old days, there was a very big and powerful country called Rome. They had lots of land and people, kind of like a huge kingdom.
Now, just like how your house has a fence around it to keep strangers out, Rome also had borders around their land. These borders were made up of walls, rivers and mountains that marked the end of their land and the start of another country's land.
The Roman Empire's borders were very long and went all around their land. The borders stretched from the big ocean in the west, all the way to the desert in the east, and even up to parts of Britain in the north.
However, the borders were not always the same. Sometimes, the Roman soldiers would have to fight with other countries to keep their land safe, and sometimes they would take over new lands and add them to their empire, which meant the borders had to be moved or extended.
So, in summary, the borders of the Roman Empire were like a big fence around their land to keep other countries out, and sometimes they had to fight to keep it strong and secure.