Modern medicine is a wonderful thing, and it gives people a commensurate increase in their ability to survive. A characters or creature that spends it's youth with access to modern medicine has their health at their first level is equal to their con stat plus the size of their first hit die. At subsequent levels, HP levels increase normally.
If they do not spend the bulk of their youth with said access, they do not gain this bonus. (though DMs are encouraged to give other bonuses instead)