Gender egalitarianism is a big phrase that means treating boys and girls, men and women equally. It's like when you and your best friend share your toys, no one gets more or less than the other one.
Imagine that you and your best friend are playing a game and you both want to be the leader. If your friend gets to lead all the time and you never get a chance, it wouldn't be fair, right? Gender egalitarianism means that everyone has equal chances and opportunities, no matter if they're a boy or a girl.
In real life, we can see gender inequality in many parts of the world. For example, some people believe that boys are better than girls and treat them differently. In some places, girls are not allowed to go to school or to do certain jobs, just because they are girls. Gender egalitarianism says that this is not fair and that everyone should be treated equally, no matter what their gender is.