The out-group homogeneity effect is the perception of out-group members as more similar to one another than are in-group members.
An in-group would be any community that you’re a part of. Any individual who isn’t a member of that community is part of the out-group. Essentially, you tend to believe that people within your community are more diverse than others.
This can lead to various stereotypes where the unique personalities of individuals are disregarded. It can even lead to serious issues such as religious differences.
We must try to avoid generalising to the most of our ability. Each and every human must be treated as an individual first, and then as a member of a community.