Have you ever seen a superhero movie where someone puts on a costume and goes out to fight crime without the police? That's kinda like what vigilantism is.
Vigilantism is when someone takes matters into their own hands to try and stop a bad person or group of people from doing bad things. They do this instead of going to the police or other people in charge to help them. It's like if you saw a bully picking on someone and you decided to go and stop the bully yourself, without talking to your teacher or your parents first.
But, vigilantism can be very dangerous and could get people into trouble. It's important to remember that the police and the justice system are there to help and protect us. So, if you see something bad happening, always talk to a grown-up and let them help you.