Willpower indicates both your ability to impose your will on others and the universe around you and also to keep your wits about you when others attempt to impose their will on you. It is an important attribute for guaging fear, steadfastness, bravery, intimidation, and many other assaults to one's mind or personality.
A Willpower of 10 means you cried at the last movie you saw, and the last movie before that.
A Willpower of 35 means you can try, but you just don't have enough bravery in you to overcome certain situations.
45 is completely average.
A character with a Willpower of 60 would keep their secrets until the thumbscrews came out, but then they just might be able to intimidate their torturers into not using them.
With a Willpower of 80, it would take weeks to break you. In fact, you could probably break them first.
A Willpower of 100 is inconceivable. You will NEVER spill your guts.