If someone in charge is telling you what to do or giving you rules to follow, and you defy that person, or defy those rules, that means you don't obey them; you don't follow their rules or orders: you ignore what they say and do your own thing, even if you get in trouble.
If your teacher tells you to sit down and be quiet, you might defy him by standing up and shouting. If your parents have a rule against running inside the house, you might defy that rule by running all over the place, even when they yell at you to stop.
In other words, to defy something is resist it, to go against it, to refuse to do as it says. A defiant child will not listen to you or do as you say. A defiant look or posture says, "Nope. You're not in charge of me. You can't tell me what to do."
Defiance can be good or bad! If the pilot of your plane is telling you to fasten your seat belt during a bumpy flight, and you defy the pilot's order? That's a bad kind of defiance, because it's dangerous. If your friend is telling you to push little kids off their swings, and you defy your friend's pressure? That's a good kind of defiance, because it's brave to stand up to a friend and do the right thing. In fact, if a parent, a teacher, a manager, or anyone else in charge tells you to do something cruel or violent, I hope you smile your most defiant smile, and say "No! I defy you!"
Most of the time, we talk about defying people and defying their rules, orders, and instructions. But we can also get abstract! If you can jump or fly really high in the air, it's like you're defying gravity; gravity is saying "Stay on the ground," but you're saying "No! I'm gonna fly!"
If you see something so wonderful or amazing that you can't even describe it or explain it, you can say that it defies description or defies explanation. That means it's so incredible that it resists being described; it refuses to be explained. How beautiful is this beach in Spain? Well, I can't even tell you, because its beauty defies description. And its beauty defies explanation.