Fairly has two distinct meanings that kids encounter all the time:

1. Treating everyone with equal respect and justice, without favoritism. When a teacher grades fairly, she judges each student's work by the same standards, not giving special treatment to her favorites. When siblings divide the last piece of cake fairly, each gets an equal share. A referee who calls a game fairly makes decisions based on the rules, not on which team he wants to win. Being fair means giving everyone a genuine chance and following the same rules for all.
2. Somewhat or moderately, more than a little but not extremely. If you say a math problem was fairly difficult, you mean it challenged you but wasn't impossible. A fairly tall building rises several stories but isn't a skyscraper. When you're fairly certain about an answer, you're confident but not completely sure. This meaning helps express the middle ground between extremes: not tiny but not huge, not easy but not overwhelmingly hard, not unlikely but not guaranteed. If your parents say you did fairly well on your report card, they're acknowledging solid work while suggesting there’s room to do even better.