Kirishitan is a very old word from a long time ago in Japan. It means "Christian" and was used to talk about people who believed in Jesus Christ and followed his teachings. But, back then in Japan, following these teachings was not allowed and could even be dangerous!
So, some people who were Kirishitan had to hide their beliefs to keep themselves safe. They might have practiced Christianity secretly in their homes or only with other people who they trusted. This was because the government at that time didn't want anyone in Japan to be Christian and they even punished people who were found to be Kirishitan.
But even with all the danger, some people didn't want to give up their beliefs. They might have risked everything to continue being Kirishitan, because their faith was important to them. Eventually, though, things changed and Japan started to allow people to practice Christianity more openly.
Nowadays, Kirishitan is still sometimes used to talk about people who are Christian in Japan, but it's not used as much as it was in the past. It's important to remember how brave the Kirishitan people were for sticking with their beliefs, even when it was really difficult!