A mesure of physical hardiness.
Usually abreviated to
CON, it's a terribly important attribute in
Dungeons and Dragons. Higher constitution means more
hit points, and this in turn means increased
survivability.
Fighters,
Monks, and
Clerics get in combat all the time, so they need it for sure. However,
rogues and
wizards have so few
hitpoints to start, they need every bonus they can get.
The moral of the story is: Make sure your CON is at least 14!