Gender discrimination is when girls and boys are treated differently just because they are a girl or a boy. For example, if a boy is told he is better at sports than a girl, even if she is actually better, just because he is a boy, or if a girl is told she can't be a scientist because she is a girl, that is gender discrimination.
It is not fair to treat someone differently just because they are a boy or a girl. Everyone should be treated equally and given the same opportunities. It is important to remember that everyone, no matter their gender, is valuable and deserves respect and fairness.