Okay kiddo, imagine you live in a place where everyone believes in a certain religion, and you believe in a different one. People think you're weird or bad for believing differently, so you have to keep it a secret. That's what happened in Nagasaki a long time ago.
There were people who believed in Christianity, which was a new religion introduced by foreign missionaries. But the rulers of Nagasaki didn't like this religion, so they made it illegal. If people got caught practicing Christianity, they would get in big trouble, maybe even get punished or killed.
So, these Christians had to be really sneaky. They couldn't go to regular churches, so they built their own hidden ones. These hidden Christian sites were usually in remote places, like in the mountains or on remote islands. They would disguise their churches by making them look like normal houses or shrines. They would only meet secretly, often at night, and they would keep everything they did and believed secret.
It wasn't easy to keep this secret, but the hidden Christians were determined to keep their faith alive. They would teach their children and pass on their beliefs in secret, knowing that they were taking a big risk by doing so. It's amazing that these hidden Christian sites have survived until today, and they are now recognized as important historical and cultural sites in Japan.