The purpose of showjumping is for horses to jump over all the combinations of fences on the course in a test of the rider's skill and the pony's/horse's power, scope, speed, athleticism, and carefulness.
Any horse should be able to jump, but there are certain breeds of horses that have a natural talent for jumping. Your horse must want to jump and you must have the skills to be able to jump your horse.
Being good at showjumping is difficult and takes many hours of practice both for the horse and the rider. Technical knowledge of jumping is critical as well as athleticism and accuracy. Having a horse that is capable is also crucial to becoming a successful showjumper.