Okay, so you know when you play with your brother or sister and sometimes you don't want them to win? That's kind of like a sports rivalry. But instead of just playing with your family, it's with teams that play sports like basketball, football, or soccer.
Sometimes teams have been playing against each other for a very long time and they have a history of not liking each other very much. This can make the games they play against each other even more exciting because they want to beat each other so badly.
Sometimes, people who like one team will make fun of people who like the other team. This is kind of like teasing in school, but it can be mean and hurtful. It's important to remember to be respectful of people who like different teams than you.
When a big game is happening between two rival teams, it can be really exciting to watch. People will often wear their team's colors and cheer really loudly. Even if you don't know all the rules of the sport, you can still have fun cheering for your favorite team!