What is a Lernaean Hydra and is it true that if you cut of one of its heads 2 more grow back?

In Greek mythology, the Lernaean Hydra was an ancient nameless serpent-like chthonic water beast (as its name evinces) that possessed seven heads — and for each head cut off it grew two more — and poisonous breath so virulent even her tracks were deadly.. The Hydra of Lerna was killed by Hercules as one of his Twelve Labours. Its lair was the lake of Lerna in the Argolid, though archaeology has borne out the myth that the sacred site was older even than the Mycenaean city of Argos since Lerna was the site of the myth of the Danaids. Beneath the waters was an entrance to the Underworld, and the Hydra was its guardian. = this is why in the movie it comes after Percy and his friends because Hades (god of the underworld) is trying to get the lightening bolt from Percy.