Discipline are set rules followed by our mind and body both like getting up early etc.
Discilpline is important as it helps in building good habits. People become responsible. Being discliplined increases person's ability, efficiency and effectiveness.
Lack of discipline result in laziness in a person. Due to which a person is not able to achieve goals. For example if a sports person is not discliplined he/she won't be able to perform well on the field resulting in loss of opportunity.