Gender are all those characteristics that differentiate a boy from a girl. These are not only physical, but also social, like the fact that boys are expected to be cool and strong, while girls should be caring and quite.
Gender equality means that boys and girls have the same rights. There shouldn’t be a difference between what boys get to have or are allowed to do and what girls get or are allowed to do. Of course there are differences between boys and girls, but this does not mean that one gender is better than the other one.