Okay, so imagine you have a really cool fort or castle that you want to protect from bad guys. Now, sometimes just having the castle isn't enough to stop the bad guys. So, what you do is you build more walls and defenses around the castle, kind of like a big ring of protection.
That's exactly what the Karelian Fortified Region was! It was a big ring of defense built around a part of Russia called Karelia. The people who built it were afraid that other countries might try to attack Karelia, so they built a whole bunch of forts, bunkers, and other defenses to make sure they could keep the bad guys out.
Now, this wasn't just some little ring of defense. The Karelian Fortified Region covered hundreds of miles! That's like walking all the way around your whole town, except with lots of walls and bunkers every few feet. It was a huge project that took a lot of time and resources to build, but it was really important to the people who lived there.
Luckily, no one ever really tried to attack Karelia through the Fortified Region, but it was still an important symbol of strength and protection for the people who lived there.