Exhaustion is measured in six levels. An effect can give a creature one or more levels of exhaustion.
Level 1: Disadvantage on ability checks. Level 2: Speed halved. Level 3: Disadvantage on attack rolls and saving throws. Level 4: Hit point maximum halved. Level 5: Speed reduced to 0. Level 6: Death.
If an already exhausted creature suffers another effect that causes exhaustion, its current level of exhaustion increases by the amount specified in the effect's description.
Finishing a long rest reduces a creature's exhaustion level by 1, provided that the creature has also ingested some food and drink.