Well kiddo, there was a really big and terrible event that happened a long time ago in a place called Nanjing, China. A war was going on between two countries, and one of them, called Japan, took over Nanjing. When they did, they did lots of bad things to the people who lived there. They hurt them, they killed them, and they even did some really terrible things to them like hurting them in ways that aren't appropriate to talk about with a five year old.
After the war was over, some people in Japan didn't want to admit that their country did something so awful to the people of Nanjing. They wanted to pretend that it didn't happen, or that it wasn't as bad as people say. That's called denial.
But, just because some people want to deny that something happened, doesn't mean it didn't happen. Lots of people know the truth about what happened in Nanjing, and they remember the people who were hurt and killed there. It's important to always remember what happened in the past, even if it's hard to think about, so we can try to make sure it never happens again.