Salvation
This may feel like an odd conversation to you. Jesus seems to be answering this man’s questions in purposefully confusing ways. The man seems to be confused as well. The man was very well educated in the Bible, so maybe Jesus is getting him to question some of his assumptions about what God wants from His followers. At the end, He makes things very clear: people must believe in Jesus to gain eternal life.
The simplicity of believing in Jesus is amazing! Like Nicodemus, we can try to understand too much instead of just believing what God has told us.