A vertical circle is a certain type of ride you may go on at an amusement park. It starts off with you sitting in a seat that is attached to a big metal track shaped like a circle that's standing straight up.
Once you're all buckled in, the ride will start to move and you will feel yourself being pulled down towards the ground due to gravity. You will then start going around the circle, and when you get to the top, you will feel like you're upside down!
But don't worry, you're not really upside down because the seat you're sitting in is keeping you from falling out. As you continue around the circle, you will eventually end up back on the ground.
It's a very exciting ride because you get to experience the thrill of going upside down without actually falling out of the seat. The ride is designed so that you stay safe the whole time.