Cystic Fibrosis (CF) is a genetic condition that affects the body's ability to pass chloride ions into and out of cells.
Yep, that's more or less all of it.
It's a complicated process, but yep, that's it.
CF patients do take extra salt.
You need to understand how genetics work.
There is a part of your genome that regulates how chloride ions pass the walls of the cells of the epithelium.
Your genome regulates how the proteins pass chloride ions through cellular membranes.
If you pass chlorides ions, the membranes pass sodium ions, and everybody is hunky dory.
If the genome makes proteins that do not accomplish this, congratulations, you have CF.
Don't misunderstand. This is the best science we have on this. The genes responsible were only identified about 1989. The mechanism where people died only shortly before that. Prior to that, people just died. Maybe your great-great-aunt died of "cholera"--it could have been what we recognize as CF.
Medicine is imprecise.