Discipline is about making progress without making mistakes as mistakes delay the process. Progress requires preparation and timely action so that it conditions both the mind and the body to gain peak results. In meditation there is no second choice or bed rest. We can only move forward and towards responsibility.
The need for discipline arises because of our human nature to be lazy. This causes procrastination and inconsistencies in our ability. Inner experiences are temporary and inconsistent in the beginning so we need to establish routine for consistency.