Okay, so imagine you have a group of friends and you all speak different languages. Let's say you speak English, but your friend speaks Russian. Now, your friend wants everyone to speak Russian instead of their own language. This is kind of what happened with Belarus and Russia.
For a long time, Belarus was its own country with its own language and culture. But then, Russia wanted to make Belarus more like Russia. So they started to encourage Belarusians to speak Russian instead of their own language. They also started to teach Russian history and culture in schools instead of Belarusian history and culture. This is called russification.
Some people in Belarus don't like this because they want to keep their own language and culture. They think it's important to remember their history and traditions. But others think it's good to be more like Russia because Russia is a bigger and more powerful country.
So basically, russification of Belarus means that Russia is trying to make Belarus more like Russia by encouraging them to speak Russian and learn about Russian history and culture. Some people like it, but other people don't.