The serpent on the pole represents that old serpent, the devil, slain & defeated by the cross
"The lifting up of the brazen serpent was to teach Israel an important lesson.
They could not save themselves from the fatal effect of the poison in their
wounds. God alone was able to heal them. Yet they were required to show their
faith in the provision which He had made. They must look in order to live. It
was their faith that was acceptable with God, and by looking upon the serpent
their faith was shown. They knew that there was no virtue in the serpent itself,
but it was a symbol of Christ; and the necessity of faith in His merits was thus
presented to their minds."
Patriarchs & Prophets p.429 E.G.W.
And the LORD said unto Moses,
Make thee a fiery serpent, and set it upon a pole:
and it shall come to pass, that every one that is bitten,
when he looketh upon
it, shall live.
Numbers 21:8
And as Moses lifted up the serpent in the wilderness,
even so must the Son of
man be lifted up:
That whosoever believeth in him should not perish,
but have eternal life.
John 3:14,5