When the people of Israel were being bitten by a plague of snakes, God told Moses to make a bronze snake as an antidote:
The LORD said to Moses, “Make a snake and put it up on a pole; anyone who is bitten can look at it and live.” So Moses made a bronze snake and put it up on a pole. Then when anyone was bitten by a snake and looked at the bronze snake, he lived. (Numbers 21:8-9 NIV)
The snake became known as “Nehushtan” (2 Kings 18:4) which is a play on the Hebrew words נחש (nachash = “snake”) and נחשת (nachoshet = “bronze”).
The bronze snake later became an object of worship and, eventually, Hezekiah had it destroyed:
[Hezekiah] … broke into pieces the bronze snake Moses had made, for up to that time the Israelites had been burning incense to it. (It was called Nehushtan.) (2 Kings 18:4 NIV)
Jesus referred to the bronze snake as an allegory of himself.
Just as Moses lifted up the snake in the desert, so the Son of Man must be lifted up, that everyone who believes in him may have eternal life. (John 3:14-15 NIV)
Just as the snakes killed the Israelites, so sin kills us. And just as looking to a bronze snake healed the Israelites, so looking to Jesus heals us from the curse of sin.