God asked Moses to make a bronze snake and hang it on a tree to show that Jesus Christ was crucified for human sins. The bitten person can live as long as he sees the copper snake, which means that the guilty person expects the redemption of the cross, while those who believe in Jesus will have eternal life.
The "snake-wrapped stick" in the Old Testament and the "cross" in the New Testament both refer to the salvation that God has prepared for people.
Galatians 3: 13 since Christ was cursed for us, he redeemed us from the curse of the law. For it is written that everyone who hangs on a tree is cursed.
I Peter 2: 24 He hanged himself on a tree and bore our sins, so that since we died in sin, we might live in righteousness. Because of his stripes, you were cured.