The Weinstein effect is when people started speaking out against the bad things that someone did to them. It’s called the Weinstein effect because it started when many women came forward and told others that they had been treated badly by a man named Harvey Weinstein who was a very powerful person in Hollywood. Many other people who had been treated badly by powerful people started sharing their stories too.
Think of it like this: if a person is doing bad things to other people, it's like a dark cloud over their head. But when many people come forward and speak out about it, it's like a big storm that starts to clear the air, and the bad things can finally be seen for what they are.