According to Wikipedia, a stereotype is a generalized belief about a particular category of people. It is an expectation that people might have about every person in a particular group.
Stereotypes are the presumption that all members of a particular group have the same traits. There was a time in Nigeria, any young man with dreadlocks was believed to be involved in internet fraud.
There are different stereotypes about gender, race, tribe and culture. They don't just happen; they're built on beliefs and experiences with certain people from a group which are then expanded to include the entire group.
The problem with stereotypes is that people are individuals, and not everyone acts the same. For example, a common gender stereotype is men do not cry, this might eventually lead to men suppressing their feelings and having poor mental health.
But stereotypes have done much worse and have even led to genocides.
The Hutus had certain stereotypes about their Tutsis that led to the genocide. They believed that Tutsis had no business living in Rwanda as they were outsiders.
They believed Tutsis continued to enjoy higher status and greater riches than Hutu notwithstanding the 1959 revolution, and they were indirectly to blame for the persistence of Hutu poverty.
So they concluded that they had a right and a duty to defend themselves since Tutsi posed a threat to them because they were always the ones who suffered, whether as a result of Tutsi military might or Tutsi cunning (using their women to woo Hutus or their money).
Hitler also believed that the Jews had power over Austria and the stock exchange, and were exploiters.
Identifying biases and assumptions about a group is the first step in dismantling harmful stereotypes. Ask yourself, "Is this true, or am I assuming?" whenever you find yourself filling in the blanks about a person without getting to know them first, you are forming an unconscious bias that is harmful.