Okay kiddo, so imagine you have two friends who don't get along and they both live in your neighborhood. One friend, let's call him Russia, decides to take over a part of the other friend's, Ukraine's, house. Russia is very bossy and likes to be in charge, so he makes Ukraine use a different flag in the part of the house that he controls.
A flag is kind of like a special symbol that represents a group of people or a country. In this case, Russia's flag looks different than Ukraine's flag, and it shows that Russia is in charge of that part of the house.
Now, some people in Ukraine really don't like that Russia is trying to control part of their house, so they don't like seeing Russia's flag there. They want Ukraine to have control of the whole house and to use their flag there instead.
So basically, the flags used in Russian-controlled areas of Ukraine are a symbol of who is in charge and there are some people who don't like it because they want Ukraine to have control again.