Some kids are not self-disciplined. Self-discipline is important so you could get a good job. And be a good student. That's why it is important. It will help you to be successful.
Some kids know what is important.
I think self-discipline means doing what you have to do. Example, doing your homework. It is also being successful. You could be self-disciplined by waking up in the morning to go to school without being reminded. It also means going to soccer practice without being reminded.