Hartal is when people protest by refusing to work or go to school. It happens when people are really mad about something and want to show the government or other people in charge that they are not happy.
It's like when you and your friends are upset because the playground is really dirty, so you refuse to play there until the principal or custodian cleans it up. Hartal is like that - it's a way for a group of people to say "we're not going to do what we normally do until things change."
Sometimes, hartal can be a peaceful way to protest and show your feelings, but other times it can lead to violence and danger. That's why it's important for people to be careful and safe when they are participating in a hartal.