Intelligence determines how well your character learns and reasons. This ability is important for wizards because it affects their spellcasting ability in many ways.
Creatures of animal-level instinct have Intelligence scores of 1 or 2. Any creature capable of understanding speech has a score of at least 3, and all Player Characters have at least 3 Intelligence.
A character with an Intelligence score of 0 is Unconscious, their mind ceasing to function as it falls into a stupor.
Some creatures do not possess an Intelligence score. Their modifier is +0 for any Intelligence-based skills or checks.