A round pen is like a circle made out of big strong poles or bars, usually made of wood or metal. Sometimes it's called a "round corral" or "arena." It's used for training and exercising horses.
When you put a horse in a round pen, they can walk, trot, or run around in circles. The walls of the round pen help keep the horse inside and focused on the person working with them. The person can stand in the middle of the pen and give directions to the horse, like telling them to stop, go left, or go right.
The round pen is a safe and controlled environment for the horse and the person working with them. It's a good place to teach young horses basic skills and manners, like how to respond to commands and behave around people.