Jinba Ittai is a Japanese word that means "horse and rider as one" or "the fusion of horse and rider." It's a cool way of saying that someone who's riding a horse is so good at it that it seems like the horse and rider are one single thing.
You know how sometimes when you're playing with a toy car or a toy truck, you can move it around really well and you feel in control of it? When someone rides a horse really well, it's kind of like that. They can tell the horse what to do, and it listens and does what they want.
Jinba Ittai is important in horseback riding because it means that the rider and the horse are in sync, which makes it easier for them to communicate with each other. That's why people who ride horses a lot work really hard to achieve Jinba Ittai.
It's like when you and your best friend do everything together, and you know each other so well that you can almost read each other's minds. That's what it's like for a horse and rider who have achieved Jinba Ittai.