“It's all about self-discipline.”
You're giving advice to your son, who's getting bad grades in school. You give him this advice to be more successful in his studies.
It's all about self-discipline.
This is something you say when you're giving someone advice on how to do something well. When you think that the key to success is just one thing, you say "it's all about ___". For example, when talking about career success, people often say:
It's all about connections.
This means that being successful in your career is mainly a result of knowing the right people. By the way, there's another version of this phrase with a similar meaning but which is more casual and sounds more negative:
It's all about who you know.
And for one more example, when giving someone advice on hitting a baseball, you can say:
It's all about keeping your eye on the ball.
This means that watching the ball carefully is the most important aspect of hitting it.
"Self-discipline" is the quality of making yourself do something that's good for you, even if you don't want to do it. It's similar to willpower, but there are some small differences:
- "willpower" is usually used to describe avoiding something that you really want to do but shouldn't. The most basic example is avoiding sweet and fatty food when you're on a diet.
- "self-discipline" is used more when talking about doing things that you're supposed to do but are hard. For "self-discipline", it will probably help to imagine someone waking up early every day to exercise.
- "willpower" is something that you can have or not have in the present moment, but "self-discipline" is something that you demonstrate over time. It doesn't make sense to say "I don't have any self-discipline today" but you can say "I don't have any willpower today."