Zero tolerance is when the rules in school say that if you do something wrong, you get in trouble no matter what. This means that even if you didn't mean to do anything wrong or didn't understand what you did was bad, you can still get punished.
For example, if you accidentally bring a toy gun to school because you didn't know it was not allowed, you could get suspended because of zero tolerance.
The reason schools have this rule is to make sure everyone is safe and knows that certain things are not allowed. However, some people think it can be too strict and not fair for kids who don't understand the rules well.