"Fair" means everyone gets an equal or fair chance to do or get something. If you have a toy that you both want to play with, and you take turns, that's fair. If you have three cupcakes and three friends, and you give each of them one, that's fair.
Sometimes things might not seem fair, like if your friend gets more candy than you, but that might be because they earned it by doing well on a test. If you both worked hard but you still think it's not fair, you can talk about it and figure out a fair solution together.
Being fair is important because it helps us all get along and be happy together. If we treat others fairly, they are more likely to treat us fairly too!