Okay, imagine you have three toys in your playroom: Estonia, Latvia, and Lithuania. Now imagine one day, a big kid named Russia comes into your playroom and takes those toys away from you. Just like that, they belong to Russia now.
That's kind of what happened in the Baltic states during the Sovietization period. Back in the 1940s, the Soviet Union wanted to control more territory and spread their ideas all around. So they invaded the Baltic states and took them over. They forced the people in those countries to follow their rules, speak their language, and live under their government.
It was kind of like if you went to your friend's house to play and their parents made you eat a food you didn't like or play a game you didn't want to play. But instead of just one family, it was a whole country forcing themselves on another.
The people in the Baltic states didn't like being taken over like that. They tried to fight back and gain their independence, but it was a really tough battle. Finally, in the 1990s, after the Soviet Union fell apart, the Baltic states were able to become independent again and have control over their own land, just like you have control over your toys in your playroom.