Fortitude is your body's ability to ward off the ill effects of sickness, disease, poison, pain, alcohol, and even death. It can be reduced over time as your character becomes ill or injured, and can be replenished through healing and rest.
A Fortitude of 10 is that of a child.
A Fortitude of 30 would indicate that your character is very sick all the time, and is quite susceptible to pain.
A Fortitude of 45 is about average.
A Fortitude of 60 would indicate that your character is a little tougher than most.
A Fortitude of 80 would class your character with a professional wrestler or a tough old biker who can take a knife in the ribs without even flinching.
A Fortitude of 100 means your character, while able to be killed through severe injury, is nearly invincible to the effects of sickness, disease, drugs, or alcohol.