Okay Kiddo, have you ever been on a playground and someone who isn't your friend comes over to play? Sometimes, we might think that everyone who isn't our friend is the same and acts the same way. This is kind of like what out-group homogeneity bias is.
So, imagine you're on a sports team and you really like everyone on your team. You might think that everyone on your team is different and special in their own way. But, if you're playing against a different team, you might think that all of the people on the other team are the same and all act the same way, even if you have never actually met them before. This is called out-group homogeneity bias because you are generalizing everyone in the other group rather than seeing them as individuals with their own unique personalities and traits.