Wolfsbane is one name for Aconitum napellus, a poisonous plant long used to kill predator animals in both Europe and India.
It plays a large part in werewolf legends and mythology.
Aconitum napellus and all its derivatives are real poisons. Ingesting and, in some cases, touching the leaves and flower can be deadly to humans. For example wolfsbane petals in fruit punch (as seen in Party Guessed) would likely kill anyone within hours of ingesting.
See Aconitum for "real world" information about the effects of the substance.
On the show it can kill werewolves, cause them to change, or heal them if used correctly.
In humans exposure can cause illness but doesn't appear to be as deadly as in the real world.
The form of wolfsbane seen in Magic Bullet, Nordic Blue Monkshood (Aconit Napel Bleu Nordique) is based on a real flower called Northern Blue Monkshood (Aconitum Noveboracense.)