Complementarianism is a big word that means boys and girls, men and women are different but equal. It's like having two different puzzle pieces that complement each other to create a bigger picture.
God made boys and girls with different traits and characteristics. Boys usually have stronger muscles and are good at things like kicking a ball or building a fort. Girls, on the other hand, are really good at things like talking and nurturing - like taking care of babies and animals. These different abilities and strengths are equally important and should be respected for what they are.
Complementarianism also means that in some cases, boys and girls have different roles in life. For example, boys can grow up to be leaders because they are usually more confident and assertive. Girls can grow up to be helpers because they are usually better at following directions and taking care of things behind the scenes.
This doesn't mean that boys are better than girls, or that girls are better than boys. They are just different, like two different kinds of flowers or animals. And both are equally important, because together, they make the world a better place!