Constitution

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.

You apply your character’s Constitution modifier to:

   Each roll of a Hit Die (though a penalty can never drop a result below 1—that is, a character always gains at least 1 hit point each time he or she advances in level).
   Fortitude saving throws, for resisting poison and similar threats.
   Concentration checks. Concentration is a skill, important to spellcasters, that has Constitution as its key ability.

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.