Every character has a base "Resilience" and "Load" that are derived from their attribute modifiers. For every skill level a character recieves in "Fortitude," the character's Resilience and Load increase correspondingly. Furthermore, once a single level of Fortitude has been purchased, characters can specialize in Resilience or Load if they so choose.
Resilience = character's ability to withstand harsh enviroments, poisons and booze. It also relates directly to the character's pain tolerance and bonus HP (when increased through the purchase of Fortitude)
Load = carrying capacity.
(see Character Creation for further info)