A character’s Constitution is a measurement of how hardy they are, how much punishment they can take, how far they can walk across difficult terrain, how they swim through freezing cold water, or how they maintain their concentration in the heat of battle.
Your Constitution score applies in the following situations:
Constitution Tests are uncommon because the endurance this ability represents is largely passive rather than involving a specific effort. Constitution is used for any attempt to physically push beyond normal limits over a period of time.
Some examples of Constitution Tests include:
When you take damage while maintaining the effects of a Concentration spell, your Constitution determines the modifier you apply when making the save.
Most importantly, your Constitution modifier contributes to your Hit Points. Hit Points are more than flesh and blood—they are also sweat, migraines, wherewithal, composure, and more besides. Players and DMs alike should view hit points as an abstraction and not the number of pints of blood in a creature’s body.
Typically, you add your Constitution modifier to each Hit Die you roll to determine the increase to your Hit Point Maximum when gaining a level or to recover Hit Points during a Short Rest, or whenever you benefit from another effect like The Fighter’s Second Wind ability.
When you create a character, at 1st level their Hit Points equal the Maximum value of their Hit Die plus their Constitution modifier.
If your Constitution modifier changes, your Hit Point Maximum also changes as if you always had the new modifier.