Basically, there are three kinds of poison: sleeping poison, paralyzing poison and normal poison. The first two will incapacitate their victim for a certain period of time, the last slowly drains health until either the victim is dead, or the poison has worn off. All kinds are part of the natural offenses some of the worlds creatures.
Protection from poison offers a chance to resist the poison and simply ignore its effects. When failed to resist a normal poison, it can still reduce the speed at which health is lost.