Ok kiddo, do you remember when we watched that really cool martial arts movie? That one where the bad guys were trying to hurt the good guys, but the good guys could stop them with just their voice? That's what a kiai is!
A kiai is a really loud shout that martial artists use when they are hitting or kicking something. It's kind of like a battle cry. They use it to help them focus and give themselves more power. When they shout, it also makes their opponent scared and confused, so they can't attack as easily.
So, next time you play pretend and you have to fight off imaginary bad guys, make sure you try a big, strong kiai! It'll make you feel like a real martial arts hero!