Racism is when people don't like other people just because of their skin color, or where they come from. Just like some kids don't like others because they have different clothes, or different toys.
In China, some people think that people who have darker skin, or who come from other countries, are not as good as they are. This makes them treat those people unfairly, or even badly.
For example, some Chinese people might not want to sit next to someone who has darker skin on the bus or subway because they think it's not clean, or they might not want to go to a restaurant if the waiters or chefs are from a different country, and have darker skin.
Racism is not a good thing. It makes people feel bad, and it doesn't let everyone have the same opportunities to do things or live a good life. It's important to treat everyone with kindness and respect, no matter where they come from, or what they look like.