The people of Israel grumbled against God and He punished them for this great sin by sending poisonous snakes amongst them. Many of the people were bitten and died.
When they saw this punishment of God come upon them, they were afraid and repented and prayed to God for forgiveness asking Him to remove the snakes.
God told Moses to make a snake of brass and put it on a pole. He promised that all who had been bitten would be healed if they looked to this brass symbol.
Many did and were healed.
We are like those people because we have sinned against God, and His punishment will come upon us because of our sins.
The Israelites could not save themselves, but the mercy of God made a way for them to be saved.
We are not sinners because we sin, but we sin because we are sinners. We can not save ourselves .But God provided a rescue, and like the Israelites who believed, looked and were healed, so the offenders who believe the word of God and look by faith to Christ on the cross will be healed for their sins.