In this passage, Jesus is explaining an Old Testament story to a teacher of the Old Testament. Although God rescued the people out of slavery, they turned on God & on Moses in the wilderness. They complained & grumbled against God & Moses. They became hardened by their cravings & desires. They became blind to God's goodness & forgot their story of God's faithfulness. Because of their rebellion, God sent snakes among them. If the people were bitten by the snakes, they would die. The only hope for healing was in looking at a bronze snake on a pole, which God instructed Moses to make. In a similar way, when someone is afflicted by deadly sin, we must look to Him  who was lifted up on the cross. His body was broken for our sins & His blood purchased a new covenant for us.