Survival outside of the cities of Athas is very difficult. Water and food are scarce.
(Most of this is covered around page 198 of the setting book, but I’ll sum up here)
A character must be supplied for each day in order not to exert themselves simply to survive. If part of the day is spent in a city, village, or oasis, supplies are not a concern unless otherwise noted.
If the character is outside a city, village, or oasis: they require supplies to survive without ill effects. This is tracked via “survival days”. A survival day is a set of supplies that will keep you healthy for one day in the desert. Traveling without supplies will cause Athas to attack vs. fortitude. Failing will cause you to contract sun sickness, which will cause loss of hit points and healing surges that cannot be regained until you are cured.
A survival day is expended every 24 hours, or after each extended rest.
Survival days can generally be purchased in cities or oases for 5gp (or looted from enemies).
You can travel by night to avoid sun sickness, but this has its own (not as severe) dangers.