Concise Bible Dictionary:
The serpent of brass that Moses made and raised on a pole when the Israelites were bitten of serpents (Num. 21:9) may, in the light of John 3:14, be regarded as symbolic of God’s way out of death into life, as well as of the condemnation of sin in the death of Christ (compare Rom. 8:3). As the bite of the serpents typified the venom of sin, and was incurable by natural means, so in the death of the Lord Jesus we see not only the ground of forgiveness of sins, but the condemnation of the state, with which sin was connected: then they who looked lived. In the history of Israel the brazen serpent came near the end of their wanderings, when their perverseness was fully manifest. In Christianity what is typified is the condemnation of sin in the flesh, as the ground of the communication of the Spirit as living water to the believer.
When the brazen serpent had become an object of worship, Hezekiah broke it in pieces, and called it Nehushtan, “a piece of brass” (2 Kings 18:4).