Mixed blood means that someone has ancestors from different racial or ethnic groups. It's like if you had a mommy who was from America and a daddy who was from Japan - you would have mixed blood because you inherited traits from both of them.
People can have mixed blood for lots of different reasons. Maybe their grandparents came from different countries, or their parents have different backgrounds. Sometimes it's because of people who moved around the world a lot, like sailors or explorers.
Having mixed blood can be really cool because it means you have different cultures and traditions to celebrate. Maybe you celebrate both Christmas and Hanukkah, or you eat spaghetti with chopsticks instead of a fork! It's important to remember that everyone is unique in their own way, and mixed blood is just one part of a person's story.