What is a stereotype?

The usual/typical/expected way a specific type of people act like (ex. Muslims, Blacks, Whites, Homeless People, Rich People, People Who Live Near The Jersey Shore, etc.).

It is usually offensive or racial to call someone a stereo type, because they are thinking that's how everyone act like who is the type/kind you are.

Some expected ways how people act like is that Nerds like comic books, Video Games, or computers a lot.

Or that saying all Emo kids cut themselves.

Some People embrace their stereotypical ways but it would be best to treat them as an average person before knowing that rather then an image that you hear or see about.
A a stereotype is a "widely held but fixed and oversimplified image or idea of a particular type of person or thing." For example many people see people of Asian ethnicity as being very smart, or that many Italians belong to a mob/gang. Many stereotypes can be very offensive to people.