Ahem, ready for a sociology lesson? OK, here we go, then. What is gender? Well, when we talk about gender, we mean the social expectations that are associated with males and females. That is, the way that women are expected to be "feminine" and men are expected to be "masculine." Wearing a skirt is something that is feminine in our culture, and a suit and tie are considered masculine. Got it? Good.
In Island of the Blue Dolphins, the gender roles on Karana's island are, at first, pretty strict. What does that mean? Well, basically that boys do certain kinds of things (hunt, make weapons) and girls do different sorts of things (take care of the home) and there's not a whole lot of overlap. But once Karana is left all alone on the island, she's forced to do the things that men only did: make weapons, hunt for food, and provide for herself. This change in Karana's life and actions shows that gender roles are not as strict as we thought along with other kinds of order on the island.
Men are from Mars and women are from Venus. They do different things because they are so different.
Gender is not strict, and our gender roles can change. Women can do things that society considers "masculine" just as men can do things thought to be "feminine."