Exhaustion is measured in 6 cumulative levels:
1: Disadvantage on Ability Checks
2: Speed halved
3: Disadvantage on Attack rolls and Saving Throws
4: Hit point maximum halved
5: Speed reduced to 0
6: Death
An effect that causes or removes exhaustion adjusts its level as specified in its description. Finishing a Long Rest reduces exhaustion by 1, as long as the creature has also ingested food and drink.