Constitution represents your character’s health and stamina. A Constitution bonus increases a character’s hit points, so the ability is important for all classes. If a character’s Constitution score changes enough to alter his or her Constitution modifier, the character’s hit points also increase or decrease accordingly.
A character with a Constitution score of 0 is Dead.
Some creatures, such as undead and constructs, do not have a Constitution score. Their modifier is +0 for any Constitution-based checks.