Okay kiddo, imagine if you had a big field that you like to play in with your friends. Sometimes, other kids from outside the field come in and try to take over or cause mischief.
This is kind of what happened a really long time ago in Croatia. There were some bad guys who wanted to take over some areas in Croatia and cause trouble.
To help protect the people in those areas, the Croatian government made a special group of soldiers called the Military Frontier. They were like the big brothers and sisters who would make sure nobody caused trouble or took over the playground.
The Military Frontier soldiers had special rules they had to follow and were given some land to live on near the areas they were protecting. They would keep watch and make sure everyone was safe.
Eventually, things got better and the bad guys went away. The Military Frontier soldiers weren't needed anymore, so they were disbanded and the land they lived on was given back to the people who used to live there.
That's the story of the Croatian Military Frontier, the special soldiers who protected some areas in Croatia a long time ago.