Willpower measures your ability to withstand psychological stress and your resistance to certain forms of mental damage. It is used to resist the effects of brainwashing, torture, seduction, telepathic probes, etc. Certain skills make use of WIL or can even boost your defense to certain forms of mental damage listed above.
It is formed by the average of Savvy and Endurance or $(SAV+END)/2$ in full. Willpower can be raised from it's base value just as any other ability, so even with a low initial value for WIL you can raise perception to be very high.