Constitution

Constitution measures health, stamina, and vital force. Characters with a high constitution score exhibit unwavering fortitude and can endure extended physical exertion, as well as resist various afflictions that might challenge their well-being.

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
• Recover from a disease
• Quaff an entire stein of ale in one go

Hit Points

Your Constitution modifier contributes to your hit points. Typically, you add your Constitution modifier to each Hit Die you roll for your hit points.
If your Constitution modifier changes, your hit point 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 hit point maximum as though the modifier had always been +2. So you add 3 hit points for your first three levels, and then add your hit points 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 hit point maximum is reduced by 7.