What is mean to train a horse?

What does it mean to train a horse? Training mean to teach a horse to do something and to respond to your request when you ask it to . It is easy to understand. 

Training should be done step by step. From simple requests to the most difficult requests . This is also true when you are teaching him a new exercise: teach him by starting with an easy request and progressing to more to difficult requests. 

Many horses seems very well trained because they look as if they can do very difficult exercises, but unfortunately the same horse might struggle to respond to your easy question.

 If a horse is well behaved and trained he must be able to do the most difficult exercises as well as  the most  basic exercises. How is possible that a well trained horse won` t stand in front of a mounting block? How is possible that a well trained horse when you try to tack him up runs to the stable corner ? Which is the answer ? 

Many riders look at what happens on the outside and they forget the inside.