A pharisee named Nicodemus came to Jesus and said, "Rabbi, we know that you are a teacher come from God, for no one can do these signs that you do unless God is with him." Jesus replied, "Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God."
Nicodemus said, "How can a man be born when he is old? Can he enter a second time into his mother's womb and be born?" Jesus answered, "Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God" (John 3:1-5).
What does it mean to be born of water and the Spirit? Some have said that water here represents physical birth, like water of the amniotic sack, and Spirit represents spiritual rebirth. Many Pentecostals teach that being born of water and spirit means you need to be baptized to be saved, and then speak in tongues to prove you have the Holy Spirit! But that's a lie.
Rather, Jesus was teaching what God had said through the prophet Ezekiel:
"I will sprinkle clean water on you, and you shall be clean from all your uncleannesses, and from all your idols I will cleanse you. And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules." Ezekiel 36:25-27
Titus 3:5 says that "He saved us, not because of works done by us in righteousness, but according to His own mercy, by the washing of regeneration and renewal of the Holy Spirit, whom He poured out on us richly through Jesus Christ our Savior."
That's what it means to be born of water and the Spirit. God cleanses us of our rebellious heart, and gives us His Spirit that we would worship and obey Him. We have nothing good to give except that which God gives to us, when we understand the text.