Okay kiddo, let me explain something really complicated in a simple way.
In the late 1930s, there was a group in the Soviet Union called the NKVD. They were like the police and secret service combined. They wanted to make sure that everyone in the country was following the same rules and ideas.
Now, the NKVD noticed that people who came from Poland (which was a country next to the Soviet Union) seemed to be different than other people in the Soviet Union. They had different ideas and ways of doing things.
So, the NKVD decided to do something called the "Polish Operation." This meant they would get rid of anyone who was from Poland or had Polish connections. They thought this would make everyone in the Soviet Union more alike.
The NKVD went around arresting Polish people and accusing them of crimes. Some of these people were really guilty of doing bad things, but others were just innocent. It didn't matter to the NKVD, they were removing anyone who was different.
Many people were sent to prison, forced to confess to crimes they didn't commit, or were executed. Families were torn apart, and some people were never seen again.
So, the Polish Operation was a terrible time in history when the NKVD tried to make everyone the same by getting rid of anyone who was different, and in doing so, a lot of innocent people suffered.