A Mandrake is a carnivorous plant which has a root that looks like a human. When unrooted, it lets out a terrible shriek that can kill non-magical humans. It is a very common ingredient for potions.
A full-grown Mandrake is depicted as a plant with a long green stem, several thorny vines used as prehensile tentacles to catch prey or enemies who dare to approach it, and a flower of bright purple colors that upon sensing prey would opens and reveal a large glowing mouth used to devour their captured prey. Young Mandrakes are yet to developed both flower and tendrils but eventually grown them as they grew larger. Given that Sucy's 5 year old Mandrake in the manga had developing flower but had no tendrils, Mandrakes would develop tentacles after they become older than 5 years.
Its roots look like a human which appearance depend on its age: young mandrakes' roots somewhat resemble a human baby while adult ones resemble a full-grown man. When unearthed, the root would release a powerful scream that can kill any ordinary human or animal who hear it as form of defensive mechanism. The lethality of the scream also depends on the age: young mandrakes would only render the victim unconscious while older ones (5 years old and upwards) can cause death in seconds.
Young roots can be used to make potions capable of restoring the magical power of a witch and the mature ones may even temporarily make her stronger than usual. But cautions must be taken when handling overflowing power from consuming potion derived from older mandrakes: Lack of control with overflowing magic would be disastrous.