Okay, so let me explain what Ukrainian SSR is like you're 5 years old.
Ukrainian SSR was a big piece of land in the country of Ukraine where a group of people called the Soviet Union had control over. It was like a big colony that the Soviet Union owned, kind of like how your parents have a house and they are in charge of it.
The Soviet Union was the boss of Ukrainian SSR and made all the big decisions. They picked who would be in charge of the government in Ukrainian SSR, and they told everyone how things should be done. It was like the Soviet Union was the parent and Ukrainian SSR was the child.
Ukrainian SSR had its own special language, Ukrainian, and its own traditions and culture. But even though they had their own special things, the Soviet Union was still making most of the big decisions.
Eventually, Ukrainian SSR became its own independent country after the Soviet Union fell apart. It was like when you grow up and get to make your own decisions without your parents always telling you what to do. That's what happened to Ukrainian SSR - they got to be their own country and make their own decisions.