John 3:5 Meaning

3Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.

4Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother's womb, and be born?

5Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.

6That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.

7Marvel not that I said unto thee, Ye must be born again.