Thirst is a measure of how thirsty you are, on a scale from 0 dehydrated to 100 fully hydrated.
You can increase your thirst value by drinking fluids or by eating some food.
It depletes slowly, 1 point every *10 seconds.
You will get a warning thirst icon when it is below 20, and if it goes down to 0 you will take 2 damage per second.
Thirst does not deplete in Global Map view.
When you revive it is going to be fully replenished.