Mixed-sex sports are when both boys and girls can play together in the same sports team. It's like when you play with your friends at school, and you can have both boys and girls on your team.
Some sports, like soccer or basketball, can be played by both boys and girls. In mixed-sex sports, the rules are the same for everyone, and everyone has to play by the same rules.
Sometimes, people can think that boys are stronger and faster than girls, but that's not always true. Girls can be just as good at sports as boys!
In mixed-sex sports, it's important to treat everyone fairly and with kindness. Boys and girls can work together and have fun playing sports, no matter what.