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!