It was a serpent inhabited by the Serpent that led humanity astray.  It was the heel of Christ that crushed the head of the serpent (Genesis 3:15).  Christ’s triumph means that the Serpent will be bound and cast into a bottomless pit.  Ironically, Jesus was lifted up on a cross in the same way that a serpent was lifted by Moses, a healing serpent meant to save (John 3:14).  On the cross Jesus defeated the Serpent.  Looking to Him on the cross heals us from the bite of the Serpent.

“And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up” John 3:14 (KJV)