In-group favoritism is when a person likes and helps other people in their own group more than people who are not in their group.
Let's say you have a group of friends you like to play with at school. You might want to pick them to be on your team in a game of kickball instead of the kids you don't know as well. This is an example of in-group favoritism because you're showing more love and care for your own friends, who are in your group, rather than people outside of it.
It's like when you're at a playground and some other kids are playing a game you don't know how to play. You might feel left out or like you're not part of their group. But when your friends come to the playground, you feel happy because they're part of your group and you get to play with them.
Sometimes, in-group favoritism can be not so fun. If you see someone being mean to a person who's not in their group, that's not okay. Just because someone is not in your group doesn't mean you should be mean to them. It's important to make everyone feel included and loved, no matter what group they're in.