Will, along with Might and Grace, are the three stats used by all characters. Will is the measure of your Willpower, which affects your magic casting capabilities. Needed mainly by Mages and Poets, it makes the successful casting of spells that much better.
For example, as a Mage, when you first get Paralyze at level 63, you'll find out very quickly that it fails... a lot. As you get to level 89 and can start to increase your will (even though the author would suggest that you wait until level 99 for this, unless you plan on being a Legends character), you will find that it gradually stops failing. Once you reach the cap of 130, it rarely fails at all.