I'm 13, but I figure this could help out anyways. Just pretend I'm 14 or something ;o
A gender stereotype is a prejudice people have for one gender or the other. For example, a gender stereotype against men is that they all love sports. A stereotype against women is that they all hate sports. That's far from true, seeing as there are many women that are huge sports fans and there are just as many men who dislike sports.
Another example is the color stereotype; pink is for girls, and blue is for boys. This also isn't necessarily true. I'm a girl and I've never been fond of the color pink, but I love the color blue. Some boys like pink but not blue.
Stereotypes of all kinds are generalizations. They can't ever be true 100% of the time, and that goes for gender stereotypes, as well.
Free~ c: Feel free to edit out parts of mine, too.