"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."