Self-discipline simply means having the ability to control yourself when you are tired or weak.
One of the hardest things to do in life is to stay focused on a task but self-discipline helps you stay focused on your goals.
Self-discipline can also mean doing activities that you do not love doing but those activities are beneficial to you.
For example, watching TV 3 hours daily does not need discipline but reading your book 3 hours daily needs discipline.
Eating 3 times a day does not need discipline but exercising 3 times a day does need discipline.
By now you should be able to understand the concept of how self-discipline works. Self-discipline is beneficial to you but it is not pleasing to your body. You need to have the willpower to resist things that are not adding value to you