Just as Moses lifted up the snake in the wilderness, so the Son of Man must be lifted up, that everyone who believes may have eternal life in him.”
The reference is to Numbers 21:8-9; the story of the bronze snake made by Moses when the Israelites were bitten by the snakes in the wilderness. I suppose this is a reference to the snake in the garden of Eden whose curse condemned mankind. The only way to be free from the curse of sin is by looking to Jesus. Note that jesus must be lifted up. There is no point in his just being there. We must make him known.