Willpower is the ability to control or restrain yourself. If you bake brownies and can keep yourself from tasting them until after dinner, you can thank your willpower.
A less descriptive — but simpler — way to say willpower is simply will. Whichever word you prefer, you can use it to describe someone's deliberate or purposeful control. In the case of willpower, the person is likely to be resisting a temptation or impulse. It takes willpower not to argue with someone whose opinions make you angry, and willpower to eat just one potato chip.