Apartheid is a bad thing that a group of people, called a government, did to some other people. It happened in a country called South Africa a long time ago.
The government in South Africa made rules that said some people were more important than others. These people were called whites, and they were given the best jobs and rights. The people who were not white were treated badly and not given the same rights and opportunities as the whites. They were treated like they were not as important.
This was wrong and against international law. International law is a set of rules and guidelines that all countries have to follow. International law says that all people should be treated the same and have the same rights, no matter what race or colour they are.
So when the South African government did these things, other countries had to act. They made rules to stop apartheid and forced the South African government to stop treating people unfairly.
Nowadays, you will hardly ever see governments that do apartheid, because it's against international law.