Heals 3 points of damage and removes the Poisoned condition. It does not restore HP lost due to Poison damage, but will stop continuous Poison-based damage that was in effect when the potion was consumed. It does not inoculate the drinker against further Poisoning.
Poison can deal HP damage (though not always), but if you fail a saving throw for Poison, under certain conditions, there may be repercussions. Simply taking Poison damage won’t trigger this condition.