A mandrake is a plant with a thick root that looks a little like a parsnip or a fat carrot. Mandrakes can be poisonous if you eat them.
Although a mandrake isn't edible, it is sometimes used in folk medicine. The root of the mandrake has very slight hallucinogenic qualities, and if it's consumed in large quantities it can cause death or coma. Mandrakes are famous in literature and folklore — they appear in the Bible, and one story claims that they scream when pulled from the ground, killing the person who harvests them.