Okay kiddo, so you know what a horse is, right? And you have probably seen big cannons before in movies or TV shows, right?
Well, horse artillery is when soldiers use horses to pull those big cannons into battle. The horses are really strong and can pull the heavy cannons over rough terrain like hills and rocks.
When the soldiers get to where they want to set up the cannons, they stop the horses and take the cannons off the wagons they were riding on. Then they set up the cannons and aim them at the bad guys they want to shoot at.
Once they light the fuse, the cannon shoots out a big, loud, deadly ball.
Horse artillery was used a lot in big battles a long time ago because it was easier to move the cannons around on horseback instead of dragging them by hand. But nowadays, armies use big trucks instead of horses to transport their artillery.