Okay, imagine you live in a really old place with walls all around your house to keep you safe. Murus Dacicus is like those walls, but even bigger and stronger. It was built by some really smart people a long, long time ago in a place called ancient Dacia (which is now Romania).
The people of ancient Dacia wanted to protect their homes and families from other people who might want to hurt them, so they built huge walls called Murus Dacicus around their cities. The walls were made from big stones stacked on top of each other, so that nobody could get in or out easily.
These walls were so strong that even if someone wanted to attack the city with big weapons or horses, they couldn't easily break through the walls. The people of ancient Dacia felt very safe and secure inside their city, thanks to Murus Dacicus.
So, in short, Murus Dacicus was a very big, strong wall that was built a long time ago by the people of ancient Dacia to protect their city from any threats.