What does John 3:14 mean?

"Lifted up" was a polite way of referring to crucifixion. That method of execution was so deliberately vile and desecrating 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 (Daniel 7:13–14). Nicodemus would have easily recognized both points, as well as the reference to the serpents,.

The Old Testament book of Numbers records the incident to which Jesus refers (Numbers 21:4–9). This event paints a vivid picture of how salvation would be brought to mankind through Christ. Israel was attacked by poisonous snakes due to their own disobedience. The people went to Moses for help. 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 was meant to foreshadow the sacrifice of Christ, as explained here (John 3:15). The people in Numbers 21 suffered because of their own failings; the 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 because of sin (Romans 5:12), and the 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 cannot 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 being rescued.