From Arkham Horror Wiki
The Will check is a type of Skill check typically presented by encounter cards. It represents a test of the Investigator's strength of mind and willpower in the situation presented in the encounter.
A Will check uses the value of the Investigator's Will skill, plus any adjustments that the Investigator would receive from skill cards, allies, and Mythos card effects, plus any adjustment specified by this specific check. Unless otherwise specified, a single success is enough to pass the check.
Note: A Will check is not the same as a Horror check and therefore cannot be improved with the use of any bonuses specific to horror checks.