On Lerna's pest th' undaunted Hero rushes,
With massy club her hundred heads he crushes,
In vain. One crush'd, two hissing heads arise,
Till good Iolas to each wound applies
The burning brand. Dipt in the Hydra's gall,
His arrows slightest wound is death to all.
The Lernaean Hydra was an ancient nameless serpent-like chthonic water beast that possessed numerous heads and poisonous breath.
As Hercules fought the creature, he cut off head after head only to have it grow back. Finally, he realized that the only way to prevent the heads from growing back was to burn the stumps with fire (or as in some stories with the poisonous blood of the creature itself). Eventually, he defeated the hydra and used its venomous blood to make poisoned arrows to use in future labours.