What is Jesus referring to when he says the following words to Nicodemus?

John 3:14

14 “Just as Moses lifted up the snake in the wilderness, so the Son of Man must be lifted up, 15 so that everyone who believes in Him will have eternal life.”

He’s referring to when the Jews were wandering in the desert. Once again, they are impatient and irritable. Once again, some of them complain. The Lord sends snakes to kill many of them as a response. Here’s the story he is referring to –

Numbers 21:4-9
4 Then they set out from Mount Hor by way of the Red Sea to bypass the land of Edom, but the people became impatient because of the journey. 5 The people spoke against God and Moses: “Why have you led us up from Egypt to die in the wilderness? There is no bread or water, and we detest this wretched food!” 6 Then the LORD sent poisonous snakes among the people, and they bit them so that many Israelites died. 7 The people then came to Moses and said, “We have sinned by speaking against the LORD and against you. Intercede with the LORD so that He will take the snakes away from us.” And Moses interceded for the people. 8 Then the LORD said to Moses, “Make a snake image and mount it on a pole. When anyone who is bitten looks at it, he will recover.” 9 So Moses made a bronze snake and mounted it on a pole. Whenever someone was bitten, and he looked at the bronze snake, he recovered.

This time is different from the many other times they have complained. Often it is Moses who intervenes, but here the people admit that they have done wrong. Often Moses steps in to avert a mass destruction of all of Israel by the Lord when they are ungrateful. This time they realize they have sinned and ask Moses to intercede with the Lord for them. The Lord tells Moses to mount an image of a snake on a pole, but they have to look at it. They have to seek it out to recover.

What Jesus is saying is that in the same way and for the same reason, he is to be mounted on a pole. Only those who admit that they have sinned and seek him will be saved from death.