What does John 3:14 mean?

"Lifted up" was a polite way of referring to crucifixion. In that culture, this method of execution was so vile that it was often mentioned using substitute phrases. "Son of Man" is a title Jesus applies often to Himself. This name refers to an Old Testament prophecy of the Messiah. Both of these points, and the reference to the serpents, Nicodemus would have recognized easily.

The Old Testament book of Numbers records the incident Jesus is referring to (Numbers 21:4–9). This incident paints a vivid picture of how salvation would be brought to mankind, through Christ. The people of Israel were attacked by poisonous snakes, as a result of their own disobedience. The people went to Moses for help, and Moses consulted with God. God instructed Moses to make an image of a snake and mount it on a pole. Anyone who looked at the snake was cured from their bite, and lived.

This event was meant to foreshadow the sacrifice of Christ, as verses 14 and 15 explain. The people in Numbers 21 are suffering as a result of their own failings, and the end result is death. Their only hope is to trust in something beyond themselves. The idea of being healed simply by looking at the bronze snake left no doubt that it was God's power, not their own, that brought healing. In the same way, all people suffer as a result of sin (Romans 5:12), and the end result is death (Romans 6:23). The only hope is trusting in something beyond ourselves (Romans 5:6). The fact that salvation comes entirely by faith leaves no doubt: we can't earn our redemption (Titus 3:5). Faith in Christ is not a "work," or something we do, any more than choosing to look at the serpent on the pole was. Both are available, to anyone, and only those who refuse to look are going to miss out on being rescued.