What does John 3:6 mean?

The Bible often describes our human nature using the term "flesh." The Greek term is sarx, which is frequently used as a reference to the sin-prone condition of human beings. It's a poetic reference to our weakness, and our hostility towards God. This is the exact opposite of the Spirit, which is from God (Philippians 3:3; John 1:33). This is the reason that a rebirth is necessary.

The phrase "born again" has become a cliché, so it means very little to most people. However, the difference between the flesh and the Spirit is the reason why rebirth is necessary. Those who are part of "the flesh" are totally incapable of loving, obeying, or knowing God. A sinful human nature makes this impossible. In order to be reunited with God, a person has to have their nature changed. This cannot happen through normal means. It requires that a person be "born again," this time in the Spirit of God.