The Haymarket Affair happened a long time ago, in 1886. It all started because workers were unhappy about how they were being treated at their jobs. They worked long hours for little pay and often in dangerous conditions. Many of them wanted to form a union, which is like a big group that can speak up for all the workers together.
Anyway, on May 3, a group of workers gathered in Haymarket Square in Chicago to protest and listen to some speeches. Suddenly, the police showed up and there was a big fight. Someone even threw a bomb, and several people were killed or injured.
The police blamed the workers and arrested eight of them. They were put on trial and some were sentenced to death, even though there wasn't much evidence that they actually did anything wrong. Many people were angry and thought the workers were being treated unfairly just for standing up for their rights.
The Haymarket Affair became an important event in the history of workers' rights and unions. People began to realize that workers deserved to be treated fairly and that they had the right to organize and make their voices heard. It's a sad event, but it helped pave the way for better working conditions and rights for workers in the future.