Constitution represents your character's health and stamina. A Constitution bonus increases a character's hit points, so the ability is important for all classes.
Constitution can be broken down into:
Fitness - A character with high fitness has great endurance and can suffer more damage than other characters while continuing to function.
Health - A character with a high health would seldom get sick and be little affected by allergies and other ailments.
You apply your character's Constitution modifier to:
* Each roll of a Hit Die (though a penalty can never drop a result below 1-that is, a character always gains at least 1 hit point each time he or she advances in level).
* The Endurance skill.
* Your Fortitude save.
4-5: Very Unhealthy - You are often sick, coughing and ravaged by the effects of pollen, mold, and multiple allergies. Many people speculate when you are going to die (often they think within the next few days!)
6-7: Unhealthy - You were perhaps a “sickly” child that's left some lingering effects on you as an adult. Your health is in horrible shape. You tire often from just crossing the room, or going up half a flight of stairs.
8-9: Below Average - You are more prone to sickness and illness than most people. When a cold or plague roll around, you often have the feeling you are going to get it (and often do!)
10-11: Average - The “standard” constitution of an adult male and female
12-13: Above Average - Your health is better than that of the average person. You sometimes get sick and maybe have one or two allergies. On a whole though, you are fairly healthy.
14-15: Healthy - You rarely get sick, maybe once every 2 years. Your fitness is at a level that it takes awhile for you get tired or exhausted.
16-17: Durable - You could count easily the number of times you've gotten sick, for you rarely do. Your fitness is a near peak level, meaning you seem to keep going, and going and going with little or no rest. You have a rather high tolerance to pain, physically.
18-19: Hardened - You can count the number of times you have been sick on one hand (if even ever!). Your body and health is at its peak. You seem to have a never-ending supply of stamina that keeps you going through the day, moving from one task to another. You have a very high tolerance to pain, physically.
20+: Legendary Constitution - Your health is the stuff myths and legends are made of. Some might wonder if you have some troll or dwarven blood in you.