Have you heard adults say things like "girls can't do math" or "boys aren't emotional"? Those are called stereotypes, which means they are generalizations about groups of people that might not be true for everyone.
The stereotype fit hypothesis is a big phrase that means people believe others fit a stereotype when they act in ways they think are typical for their group. For example, if a girl does really well in math, people might be surprised because they think girls are bad at it. But if a boy does well in math, people might not be surprised because they think boys are good at it.
Sometimes we make assumptions based on stereotypes without realizing it, and that can be unfair to people who don't fit those assumptions. It's important to remember that everyone is unique and should be judged based on their individual abilities and actions, not on what group they belong to.