Curtsy is a special way to show respect and honor to people who are important to us, like monarchs (kings and queens), leaders, or even our parents and grandparents. It's like saying "hello" or "thank you", but instead of using our words, we use our bodies.
To curtsy, you stand up straight and tall, then take one foot and cross it behind the other foot. Next, you bend your knees and lower your body down until one knee touches the ground (like you're sitting down on one knee). At the same time, you take your hands and fold them in front of you, almost like you're hugging yourself.
When you're done, you stand back up and put your feet back in their normal position. It's kind of like a fancy dance move that you do to show respect and honor to someone who's important.