Simply, stereotypes are an oversimplified generalisation of a person, thing or group. There are many stereotypes, many of which are related to the different racial groups. Because of this people could be misjudged even before they have been introduced.
These are stereotypes based on a person's race, ethnicity and nationality. For example, that asians are good at math.
These are stereotypes based on a person's gender. For example, that boys like blue and girls like pink.
These are stereotypes based on a person's age. For example, that all elderly are incapable.
Stereotypes are bad because they create negative and untrue images of certain groups of people. Every group of people are diverse, and include many different personalities and different people, making it extremely unfair to certain people who are thought to be unable to do certain things because of certain differences such as gender, age or race.