5e SRD:Constitution

Constitution
Constitution measures health, stamina, and vital force.

Constitution Checks
Constitution checks are uncommon, and no skills apply to Constitution checks, because the endurance this ability represents is largely passive rather than involving a specific effort on the part of a character or monster. A Constitution check can model your attempt to push beyond normal limits, however.

The GM might call for a Constitution check when you try to accomplish tasks like the following:


 * Hold your breath
 * March or labor for hours without rest
 * Go without sleep
 * Survive without food or water
 * Quaff an entire stein of ale in one go

Hit Points
Your Constitution modifier contributes to your. Typically, you add your Constitution modifier to each Hit Die you roll for your hit points.

If your Constitution modifier changes, your maximum changes as well, as though you had the new modifier from 1st level. For example, if you raise your Constitution score when you reach 4th level and your Constitution modifier increases from +1 to +2, you adjust your maximum as though the modifier had always been +2. So you add 3 for your first three levels, and then roll your  for 4th level using your new modifier. Or if you’re 7th level and some effect lowers your Constitution score so as to reduce your Constitution modifier by 1, your maximum is reduced by 7.