Here is a evangelical view from the Protestant tradition.
The concept of 'birth' means something that happens to you, not as a result of your own work, effort or commitment. The doctrine of new birth pertains to the new life one has the moment they believe the gospel.
The necessity for new birth shows that the old life from Adam can't be improved. It must be completely killed and destroyed.
There Jesus was teaching a highly moral religious leader that to obtain eternal life we must die and be reborn.
This 'new birth' is spiritual as opposed to our natural birth. In our natural birth we are born sinners. By faith in Christ we are adopted, removed form our first family under the kingdom of the devil, and tranlsated into Chrit's kingdom.
This new birth entirely changes the way we live.
The whole subject can be restated by Romans:
Life Through the Spirit
1 Therefore, there is now no condemnation for those who are in Christ Jesus, 2 because through Christ Jesus the law of the Spirit who gives life has set you free from the law of sin and death. 3 For what the law was powerless to do because it was weakened by the flesh, God did by sending his own Son in the likeness of sinful flesh to be a sin offering. And so he condemned sin in the flesh, 4 in order that the righteous requirement of the law might be fully met in us, who do not live according to the flesh but according to the Spirit. (NIV Romans 8:1-4)
The same ideas is expressed in Ephesians:
Made Alive in Christ
1 As for you, you were dead in your transgressions and sins, 2 in which you used to live when you followed the ways of this world and of the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient. 3 All of us also lived among them at one time, gratifying the cravings of our flesh and following its desires and thoughts. Like the rest, we were by nature deserving of wrath. 4 But because of his great love for us, God, who is rich in mercy, 5 made us alive with Christ even when we were dead in transgressions —it is by grace you have been saved. 6 And God raised us up with Christ and seated us with him in the heavenly realms in Christ Jesus, 7 in order that in the coming ages he might show the incomparable riches of his grace, expressed in his kindness to us in Christ Jesus. (NIV Ephesians 2:1-7)
New birth is not a new term.
Martin Luther said:
You have been born anew. In the third place, this should be done because you are no longer what you formerly were, he says, but are new persons. Works have not brought this about, but for it a birth has been required. For you cannot make the new man. No, he must grow or be born. Just as a carpenter cannot make a tree, but the tree itself must grow out of the earth, and just as we all were not made children of Adam but were born as such and have inherited sin from our father and mother, so we cannot become children of God by means of works but must be born again. This is what the apostle wants to say: Since you are now new creatures, you must also conduct yourselves differently now and lead a new life. Just as you formerly lived in hatred, so you must now walk in love, contradictory in every respect. (Luther's Works Volume 30, Page 43)
John Calvin said:
By the phrase born again is expressed not the correction of one part, but the renovation of the whole nature. Hence it follows, that there is nothing in us that is not sinful; for if reformation is necessary in the whole and in each part, corruption must have been spread throughout.
This expression shows briefly what is the beginning of Christianity, and at the same time teaches us, that we are born exiles and utterly alienated from the kingdom of God, and that there is a perpetual state of variance between God and us, until he makes us altogether different by our being born again; for the statement is general, and comprehends the whole human race. (Calvin's Commentary on John 3:3)
Those who espouse this term usually mean that committed people in churches (similar to Nicodemus) are still damned, even if they are a minister, priest, have been baptized, go to church, do good works, read the bible, share the gospel, pray, give to the poor, etc. ------> until they have a new birth through genuine faith in the gospel.