So imagine you have a toy car that you really love. One day, someone comes and takes the car away from you without asking permission. They start playing with your toy car and even say that it's their toy now, even though it really belongs to you.
This is kind of what happened in Ukraine. There are certain areas in Ukraine called the "occupied territories" where some people came and took control of those areas without asking permission from the Ukrainian government. These areas are called Crimea and parts of eastern Ukraine.
Now, the people who came and took control of these areas said that they wanted to be separate from Ukraine and have their own little country. But Ukraine didn't agree with that because those areas are a part of Ukraine, just like your toy car belongs to you.
Since then, Ukraine and the people in the occupied territories have been having some disagreements and conflicts. Ukraine wants those areas back, just like you would want your toy car back. But it's not easy because those people are not willing to give the areas back and it's causing a lot of problems and tension between them.
So, to sum it up, the occupied territories of Ukraine are areas where some people took control without asking, and Ukraine wants those areas back because they are a part of Ukraine. It's like when someone takes your toy car without permission and you really want it back.