From today's reading: And just as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, that whoever believes in him may have eternal life.” John 3:15
I'm sure the disciples were asking, "What is he talking about?" All the way through - He gave his disciples hints about what was going to happen. But things were going so well and they didn't want to think about any unpleasantness.
In the case of Moses - looking at the serpent on the pole was the cure for those bitten by serpents - they would look upon it and be saved. In the same way, Jesus is the cure for our sinfulness - anyone looking upon the image of Jesus lifted up on the cross and believing in him will be saved. As Jesus said to the thief on the cross, "Today you will be with me in paradise." Jesus says that to each of us -
Like Nicodemus, many of us initially come to Jesus at night - when the world seems darkest, Jesus is the one who can shine light into our darkness - and Jesus is the one who can see us through that darkness and bring us out into the light. He bids each of us to come - see him and trust him and know that he watches over us.